Ventilation Age Research Articles

Deep water ventilation in the northwestern North Pacific during the last deglaciation and the early Holocene (15-5 cal. kyr B.P.) based on AMS 14C dating

The difference between benthic and planktonic foraminifera radiocarbon (B–P 14C) age differences in core PC6 (40° 23.89′ N, 143° 29.87′ E) retrieved from the northwestern North Pacific provide a clue to the reconstruction of deep water circulation during the last deglaciation and the early Holocene (15-5cal.kyrB.P.). The observed B–P 14C age differences ranged from 1030 to 1630 years, which are comparable to the present-day apparent ventilation age. It suggested that the ventilation generally remained similar during 15-5cal.kyrB.P. However, B–P 14C age difference slightly reduced at 14.6cal.kyrB.P., indicating that the higher ventilation temporality could have occurred in the northwestern North Pacific (∼2200m water depth).

Seasonal Variations in Peruvian Marine Reservoir Age from Pre-Bomb Argopecten Purpuratus Shell Carbonate

Marine upwelling along coastal Peru can be intense and variable, making radiocarbon dating marine and coastal systems complex. Historical and proxy records of upwelling along coastal Peru are few, and long-lived species such as corals do not grow in the cold coastal waters. Mollusk shell carbonate, however, can record both the magnitude of the local marine reservoir correction, ΔR, and of seasonal oscillations in the ventilation age of coastal waters. If large, these seasonal oscillations would complicate radiocarbon dating of marine organisms. To examine this possibility, we sampled for δ13C, δ18O, and 14C content a set of pre-bomb Argopecten purpuratus shells collected from coastal Peru during 1908 and 1926. Intrashell variations of up to 216 14C yr were noted, but these were not consistently correlated with seasonal changes in δ18O or δ13C. Only an 11 yr difference was observed in the weighted average ΔR of Callao Bay shells collected during normal (1908) and El Niño (1926) years. Despite the intrashell 14C variation noted, weighted average ΔR values from all 3 sample sites and from normal and El Niño years all overlap at 1 σ. We report ΔR values of 183 ± 18 and 194 ± 23 yr from Callao Bay (12°4′S), 165 ± 24 yr from Salaverry (8°14′S), and 189 ± 23 yr from Sechura Bay (5°45′S).

Origin of the deep Bering Sea nitrate deficit: Constraints from the nitrogen and oxygen isotopic composition of water column nitrate and benthic nitrate fluxes

On the basis of the normalization to phosphate, a significant amount of nitrate is missing from the deep Bering Sea (BS). Benthic denitrification has been suggested previously to be the dominant cause for the BS nitrate deficit. We measured water column nitrate 15N/14N and 18O/16O as integrative tracers of microbial denitrification, together with pore water‐derived benthic nitrate fluxes in the deep BS basin, in order to gain new constraints on the mechanism of fixed nitrogen loss in the BS. The lack of any nitrate isotope enrichment into the deep part of the BS supports the benthic denitrification hypothesis. On the basis of the nitrate deficit in the water column with respect to the adjacent North Pacific and a radiocarbon‐derived ventilation age of ∼50 years, we calculate an average deep BS (>2000 m water depth) sedimentary denitrification rate of ∼230 μmol N m−2 d−1 (or 1.27 Tg N yr−1), more than 3 times higher than high‐end estimates of the average global sedimentary denitrification rate for the same depth interval. Pore water‐derived estimates of benthic denitrification were variable, and uncertainties in estimates were large. A very high denitrification rate measured from the base of the steep northern slope of the basin suggests that the elevated average sedimentary denitrification rate of the deep Bering calculated from the nitrate deficit is driven by organic matter supply to the base of the continental slope, owing to a combination of high primary productivity in the surface waters along the shelf break and efficient down‐slope sediment focusing along the steep continental slopes that characterize the BS.

How accurate is the estimation of anthropogenic carbon in the ocean? An evaluation of the ΔC* method

The ΔC* method of Gruber et al. (1996) is widely used to estimate the distribution of anthropogenic carbon in the ocean; however, as yet, no thorough assessment of its accuracy has been made. Here we provide a critical re‐assessment of the method and determine its accuracy by applying it to synthetic data from a global ocean biogeochemistry model, for which we know the “true” anthropogenic CO2 distribution. Our results indicate that the ΔC* method tends to overestimate anthropogenic carbon in relatively young waters but underestimate it in older waters. Main sources of these biases are (1) the time evolution of the air‐sea CO2 disequilibrium, which is not properly accounted for in the ΔC* method, (2) a pCFC ventilation age bias that arises from mixing, and (3) errors in identifying the different end‐member water types. We largely support the findings of Hall et al. (2004), who have also identified the first two bias sources. An extrapolation of the errors that we quantified on a number of representative isopycnals to the global ocean suggests a positive bias of about 7% in the ΔC*‐derived global anthropogenic CO2 inventory. The magnitude of this bias is within the previously estimated 20% uncertainty of the method, but regional biases can be larger. Finally, we propose two improvements to the ΔC* method in order to account for the evolution of air‐sea CO2 disequilibrium and the ventilation age mixing bias.

Radiocarbon and stable isotope constraints on Last Glacial Maximum and Younger Dryas ventilation in the western North Atlantic

Foraminiferal abundance, 14C ventilation ages, and stable isotope ratios in cores from high deposition rate locations in the western subtropical North Atlantic are used to infer changes in ocean and climate during the Younger Dryas (YD) and Last Glacial Maximum (LGM). The δ18O of the surface dwelling planktonic foram Globigerinoides ruber records the present‐day decrease in surface temperature (SST) of ∼4°C from Gulf Stream waters to the northeastern Bermuda Rise. If during the LGM the modern δ18O/salinity relationship was maintained, this SST contrast was reduced to 2°C. With LGM to interglacial δ18O changes of at least 2.2‰, SSTs in the western subtropical gyre may have been as much as 5°C colder. Above ∼2.3 km, glacial δ13C was higher than today, consistent with nutrient‐depleted (younger) bottom waters, as identified previously. Below that, δ13C decreased continually to −0.5‰, about equal to the lowest LGM δ13C in the North Pacific Ocean. Seven pairs of benthic and planktonic foraminiferal 14C dates from cores >2.5 km deep differ by 1100 ± 340 years, with a maximum apparent ventilation age of ∼1500 years at 4250 m and at ∼4700 m. Apparent ventilation ages are presently unavailable for the LGM < 2.5 km because of problems with reworking on the continental slope when sea level was low. Because LGM δ13C is about the same in the deep North Atlantic and the deep North Pacific, and because the oldest apparent ventilation ages in the LGM North Atlantic are the same as the North Pacific today, it is possible that the same water mass, probably of southern origin, flowed deep within each basin during the LGM. Very early in the YD, dated here at 11.25 ± 0.25 (n = 10) conventional 14C kyr BP (equal to 12.9 calendar kyr BP), apparent ventilation ages <2.3 km water depth were about the same as North Atlantic Deep Water today. Below ∼2.3 km, four YD pairs average 1030 ± 400 years. The oldest apparent ventilation age for the YD is 1600 years at 4250 m. This strong contrast in ventilation, which indicates a front between water masses of very different origin, is similar to glacial profiles of nutrient‐like proxies. This suggests that the LGM and YD modes of ocean circulation were the same.

Rapid transient changes in northeast Atlantic deep water ventilation age across Termination I

A sequence of accelerator mass spectrometry (AMS) 14C dates performed on benthic and planktonic foraminifera from a northeast Atlantic deep‐sea core (MD99‐2334K; 37°48′N, 10°10′W; 3146 m) permit the reconstruction of deep water “14C ventilation ages” across the last deglaciation. The records from MD99‐2334K have been placed on the GISP2 timescale via the synchrony of temperature changes recorded in the Greenland ice cores and in North Atlantic planktonic δ18Occ (calcite δ18O). On the basis of a range of estimates for past source water Δ14C, this permits the estimation of 14C projection ventilation ages for comparison with benthic‐planktonic 14C age differences. Although the accurate estimation of past ventilation ages is precluded by unknown deep water Δ14C source signatures, and by uncertainty regarding the extent of deep water mixing, it is clear that deep water ventilation in the northeast Atlantic was significantly reduced during the last glaciation, increased abruptly coincident with the Bølling‐Allerød warming, and rapidly became reduced again during the Younger Dryas cold reversal. The character of these changes is consistent with a varying dominance of North Atlantic Deep Water (NADW) versus Antarctic Bottom Water (AABW). Parallel benthic δ13C, deep water temperature (Tdw), and deep water δ18O (δ18Odw) estimates support this inference. The fact that deglacial changes in the deep water radiocarbon content of the northeast Atlantic run parallel to opposite changes in atmospheric radiocarbon content, and in parallel with Greenland temperature fluctuations, unequivocally implicates changes in ocean circulation in deglacial climate evolution and illustrates the capacity for the deep ocean to respond and contribute to abrupt climate change.

Applying the new TrOCA approach to assess the distribution of anthropogenic CO 2 in the Atlantic Ocean

Based on the recently proposed composite tracer ‘TrOCA’, a novel and independent approach is developed to estimate the concentration of anthropogenic CO 2 in marine systems. It is shown that the precision of this simple model can be as high as 3 to 5.9 μmol kg −1. Several assumptions currently used in some previous back-calculation methods become irrelevant by using the new tracer TrOCA (e.g. constant air–sea disequilibrium, negligible diapycnal mixing, and/or ventilation age of water masses deduced from anthropogenic tracers). The model is then applied throughout the Atlantic Ocean to estimate the concentrations of anthropogenic CO 2, using only the four properties TCO 2 (dissolved inorganic carbon), TA (total alkalinity), O 2 (oxygen), and θ (potential temperature), all available from the CARINA database. The results, which are computed for the 1980s and the 1990s, are in good agreement with the distribution of three independent anthropogenic tracers (Δ 14C, 3H, and CFC-11). The anthropogenic CO 2 inventory within the Atlantic Ocean (80°N–50°S) reaches ∼40 gigatons of carbon (Gt C) in the 1980s, and it increases up to ∼45 Gt C during the 1990s. From one decade to another, the largest increase of inventory occurs in the southern Atlantic Ocean (33.6%), whereas in the northern basin it reaches only 4.3%. The southern Atlantic Ocean is currently playing an important role in sequestrating a large fraction of the excess CO 2. The Atlantic Ocean is storing anthropogenic CO 2 at a net rate of 0.66 Gt C year −1. Extrapolating this estimate to the world ocean, a total of 1.97 Gt C year −1 could have been effectively stored in the ocean interior over the period 1984–1995.

Dissolved Al, In, and Ce in the eastern Indian Ocean and the Southeast Asian Seas in comparison with the radionuclides 210Pb and 210Po

To carry out comparative geochemical investigation of refractory and reactive metals in different oceanic settings covering different θ-S characteristics, productivity, dissolved oxygen profiles, water and sediment discharge, etc., we have determined the vertical profiles of dissolved (<0.04 μm) Al, In and Ce, as well as 210Pb and 210Po in the eastern Indian Ocean (from 40°S in the Southern Ocean to 8°N in the Bay of Bengal) and the Southeast Asian Seas. In the Antarctic Circumpolar Region, the concentrations of these refractory metals are very low, presumably due to very low the atmospheric input and intensified scavenging. Resemblance in the vertical profiles of these metals is often seen in some other stations. However, there are also significant differences among their distributions, for example, in the magnitude of surface enrichment caused by the external input from eolian and fluvial-coastal sources. Comparison of Al distributions in surface waters with those of atmospherically derived 210Pb suggests the relative importance of eolian input over fluvial-coastal sources. Fluvial and coastal input appears to be insignificant for dissolved In, but may be important for Ce. The mean residence time of Al in the surface mixed layer was estimated to be ∼2 years which is similar to that of 210Pb. In the intermediate and deep waters, the concentrations of each element vary with depth and location. The range of variation is in the order of Al>Ce>In, depending upon particle reactivity. Although dissolved Al decreases along the water trajectory by particle scavenging, variations of dissolved In and Ce are relatively small which may be due to less scavenging for both elements. Compared with significantly high (>4 pM) dissolved Ce throughout the water column in the Bay of Bengal, dissolved Al concentration remains low, suggesting that it has higher affinity to particles and hence is scavenged by sinking particulate matter. This is consistent with the observation that the dissolved Al in the Antarctic Intermediate Water (AAIW) decreases from 4 to 6 nM in the 30°S Perth Basin to <0.7 nM in the 10°S West Australia Basin along its trajectory. Using the chlorofluorocarbons (CFCs) ventilation age of AAIW (Fine, 1993), the mean residence time of Al in the intermediate and deep waters in the eastern Indian Ocean is estimated to be <17 yr, approximately the same as that of 210Pb (10–15 yr). In the semiclosed basins of Southeast Asia, the distributions of Al, In and Ce are also very unique. In the South China Sea, there is a strong sediment source for dissolved In and Ce during the deepwater passage through the Luzon Strait.

Ocean ventilation and sedimentation since the glacial maximum at 3 km in the western North Atlantic

Stable isotope, sedimentological, and radiocarbon data from cores at ∼3 km water depth on the Blake Ridge, western subtropical North Atlantic, reveal the history of deep water ventilation since the last glacial maximum (LGM). Bulk sediment accumulation rates varied locally by a factor of 2 under the influence of bottom currents in this sediment drift environment, but the sand flux, mostly foraminifera, was nearly identical at a given time. This suggests that the rain rate of foraminifera (mostly planktonic) was constant, that transport of foraminifera was negligible, and that current‐controlled differences in clay and silt transport drive bulk accumulation. In two of the cores, flux peaks in the benthic foraminifera Cibicidoides and Uvigerina peregrina occurred during the Younger Dryas (YD) cold event, and at 18.2, 19.6, 21.1, 25.0, and 28.1 ka. Radiocarbon measurements on those benthic foraminifera show the ventilation age of bottom waters was ∼1000 years during the YD, and for older events it was as great as 2000 years. These results contrast with Holocene ventilation, which was ∼500 years and 700 years at 1500 years and ∼7100 years before present, respectively.

Really Deep Breathing

Deep-sea mixing (ventilation) rates depend on how fast deep sea water is formed. The deep sea contains more than 95% of the carbon in the ocean-atmosphere system, so the ventilation rate directly affects atmospheric composition and radiative forcing. Goldstein et al. present a suite of radiocarbon and uranium-series dates of deep-sea corals from the Southern Ocean and derived rates of deep-sea ventilation for the present and the last glacial period (about 16,500 years ago). They calculate that the ventilation age then was 20 to 40% greater than at present. This result is consistent with reported findings from the Atlantic and Pacific Oceans and suggests that glacial oceans mixed more slowly than today's oceans. Thus, glacial-age, carbon-rich deep water may have mixed less quickly with surface water and exchanged CO2 less rapidly with the atmosphere. Slower mixing would have significantly lowered the partial pressure of atmospheric CO2 during that period.—HJS

Uranium-series and radiocarbon geochronology of deep-sea corals: implications for Southern Ocean ventilation rates and the oceanic carbon cycle

We present new uranium-series and radiocarbon measurements for deep-sea corals from the Southern Ocean. These data are used to reconstruct ventilation ages, both at present and at the end of the last glacial period approximately 16 500 years ago. We apply an improved two-component mixing approach to correct uranium-series dates for contaminant thorium and protactinium present in oxide coatings. Calculated seawater radiocarbon values for contemporary samples decrease with depth in the water column and agree with direct seawater radiocarbon measurements for this area. This indicates that deep-sea corals can accurately record seawater radiocarbon distributions. Two of three glacial samples experienced open-system uranium-series systematics, however, a third sample from the Drake Passage yields concordant thorium and protactinium dates as well as seawater values for initial 234U/ 238U. This coral yields a ventilation age that is approximately 20–40% greater than modern values for its location. This increase is consistent with published deep-sea coral and calibrated planktonic–benthic foraminifera radiocarbon data, suggesting that the glacial oceans as a whole may have been substantially less ventilated, presumably due to decreased formation of North Atlantic Deep Water. An overall decrease in oceanic mixing rates could have contributed to lower dissolved carbon in surface ocean water and lower atmospheric pCO 2 during the past glacial period.

Determination of Persian Gulf Water Transport and oxygen utilisation rates using SF6 as a novel transient tracer

Sulphur hexafluoride (SF6) has potential as a transient tracer of recently ventilated water masses, as its atmospheric burden continues to increase. Northern Arabian Sea hydrography was examined using measurements of atmospheric and dissolved SF6, CFC‐11, CFC‐12 and CFC‐113. Persian Gulf Water (PGW) was characterised by its SF6 signal, and the time elapsed since its formation was evaluated by two approaches. Four ventilation age estimates were derived from SF6/CFC‐11, SF6/CFC‐12, CFC‐113/CFC‐11 and CFC‐113/CFC‐12, and their agreement at the oceanic stations confirms the validity of SF6 as a transient tracer. A second approach, of correcting SF6 partial pressure for PGW dilution by an optimal mixing model and referencing to the atmospheric SF6 chronology, provided a relative tracer age. This indicated a PGW flow of 0.016 (+/−0.003) m/s across the northern Arabian Sea, with an associated oxygen consumption of 10.1 µmol/l p.a. that exceeds tracer‐derived estimates but confirms rates derived from export flux.

Pulmonary antioxidant enzyme activity during early development: Effect of ventilation.

OBJECTIVE: To examine the effect of tidal liquid ventilation (TLV) and conventional gas ventilation (GV) on the pulmonary antioxidant enzyme (AOE) activity in two groups of preterm lambs at 110 and 120 days of gestation and to compare these results to age-matched fetal controls. DESIGN: Experimental, prospective, randomized, controlled study. SETTING: School of medicine, department of physiology. SUBJECTS: Thirty-two premature lambs at 110 days (n = 16) and 120 days (n = 16) gestation. INTERVENTIONS: Six lambs at 110 and 120 days were ventilated with TLV for 4 hrs. Three lambs at 110 days were ventilated with GV for 3 hrs, and six lambs at 120 days were ventilated with GV for 4 hrs. Four lambs, each at 110 and 120 days, were used as age-matched fetal controls. Measurements: Sequential measurements of arterial blood chemistries were performed in all groups. Biochemical assays included catalase activity, superoxide dismutase activity, and glutathione peroxidase activity. Histologic examinations of lung sections from TLV and GV lungs were performed. Main Result: Despite vast differences in physiologic outcome, there were fewer differences in AOE activity as a function of ventilation (fetal control, GV, and TLV lambs) and/or gestational age. There were no significant differences in superoxide dismutase and glutathione peroxidase activity as a function of age or ventilation. Catalase activity was significantly higher (p <.05) in fetal 120-day control lambs (24.8 +/- 1.9 units/mg protein) relative to the 110-day control lambs (14.3 +/- 1.7 units/mg protein). Catalase activity significantly (p <.05) decreased in both GV and TLV 120 day lambs compared with fetal controls; yet, in the 110-day gestational lambs, there were no significant differences in enzyme level as a function of ventilation. Thus, an age-specific response to ventilation was evident for catalase activity. CONCLUSION: The present data demonstrate several developmental differences in AOE activity. The lack of ventilation effect in AOE activity, in view of the big difference in physiologic and inflammatory outcomes, suggests that mechanisms associated with free radicals do not underlie the response to different modes of ventilation.

Constant ventilation age of thermocline water in the eastern subtropical North Pacific Ocean from chlorofluorocarbon measurements over a 12‐year period

Northeastern Pacific chlorofluorocarbon (CFC) data collected between 1982 and 1994 near Geochemical Ocean Sections Study (GEOSECS) station 1 (28.5°N, 122.5°W) record decadal timescale ventilation processes of the subtropical thermocline in this region. The CFC‐12 concentration age field versus potential density has been remarkably constant over the 12‐year period, although CFC concentrations in the upper kilometer of the water column have increased with time. Results from a simple one‐dimensional advection‐diffusion model are consistent with an advection velocity of ca. 0.8–0.9 cm s−1 from the source area. The influence of the 1982–83 El Niño is noticeable in the 1983 observations. While the main stream of subarctic source water appears to spread southward at a constant rate, during El Niño years the influence of comparatively CFC‐free tropical thermocline waters is enhanced in this region, leading to reduced vertical inventories of CFCs, but without changing the apparent CFC ages as functions of potential density. Apparent oxygen utilization rates decrease with increasing CFC age, and also appear not to have changed significantly over the 12‐year measurement period.

The flow of Antarctic bottom water to the southwest Indian Ocean estimated using CFCs

Observations of chlorofluorocarbons (CFCs), carbon tetrachloride, temperature, and salinity from five sections following the outflow path of Antarctic Bottom Water (AABW) into the southwest Indian Ocean are reported. The transient tracer data clearly show the plume of recently ventilated water whose hydrographic properties are progressively altered by mixing with the overlying waters. We use the CFC measurements to estimate the mean speeds (or transit times) and mixing rates (or dilutions) of the abyssal flow at each section using simple kinematic circulation models. Given our assumptions, the CFC ventilation age equals the transit time. The results suggest a transit time of 23±5 years (outflow speed of 1.2±0.3 cm s−1) to the Crozet‐Kerguelen Gap with a dilution of 8–15 from the surface waters of the Weddell Sea. The estimated horizontal diffusivity is 30–70 m2 s−1, and the vertical diffusivity is 3–7×10−4 m2 s−1 Combined with the estimate of R. R. Dickson (unpublished data, 1998) for the AABW transport at this point, we conclude that a volume flux of 0.8–1.6 Sv (106 m3 s−1) is leaving the continental shelves of the Weddell Sea to eventually enter the abyssal Indian Ocean past Crozet Island.

Observations of temporal changes of tritium-&lt;SUP&gt;3&lt;/SUP&gt;He age in the eastern North Atlantic thermocline: Evidence for changes in ventilation?

A compilation of fifteen years of tritium and 3 He measurements is used to examine the ventilation of the eastern North Atlantic subtropical gyre with specific emphasis on the temporal character of the tracer age field. A multivariate regression analysis in the form of a spatiotemporal Taylor expansion is applied to observations interpolated onto isopycnal surfaces. The time-dependent component of the tracer age field is found to be statistically significant, explaining approximately 10% of the variance of the tracer age observations in the upper thermocline (σ θ = 26.5) and increasing to roughly 50% of the variance in the lower thermocline (σ θ = 27.0). The observed transient tracer age increases over the 15 years of observations with the fractional rate of change of the age field varying between 2% and 5% per year. The largest observed changes occur on the deepest, most slowly ventilated isopycnal surfaces. The second derivative of the tritium- 3 He age with time suggests that the tracer age field may be approaching a steady state. If tritium- 3 He age is interpreted as a true measure of the advective ventilation age, the temporal changes in age would imply a slackening of the ventilation of the lower main thermocline of greater than 50% from the late 1970's to the early 1990's. However, consideration of the full advective-diffusive balance of tritium- 3 He age reveals that the changes in tracer age field represent a time-dependent adjustment of the transient tracer concentrations in conjunction with a steady local circulation field. Integral approximations of the upstream evolution of the tracer field also fail to demonstrate evidence for decadal changes in ventilation. The integral balance along the path of subduction yields an improved estimate of the true ventilation age based on the temporal tendency of the age field along the path of ventilation. An approximation of this integral suggests that actual ventilation ages can be up to 40% larger than the measured tracer age in the deeper portions of the North Atlantic thermocline. Proper accounting of the time-dependent biases of the tracer age dating technique are a prerequisite for examining transient tracer measurements for evidence of changes in the physical ventilation of the upper ocean.

Changing atmospheric Δ14C and the record of deep water paleoventilation ages

We propose a new calculation method to better estimate the deep water ventilation age from benthic‐planktonic foraminifera 14C ages. Our study is motivated by the fact that changes in atmospheric Δ14C through time can cause contemporary benthic and planktonic foraminifera to have different initial Δ14C values. This effect can cause spurious ventilation age changes to be interpreted from the geologic data. Using a new calculation method, 14C projection ages, we recalculate the data from the Pacific Ocean. Contrary to previous results, we find that the Pacific intermediate and deep waters were about 600 years older than today at the last glacial maximum. In addition, there are possible signals of ventilation age change prior to ice sheet melting and at the Younger Dryas. However, the data are still too sparse to constrain these ventilation transients.

High precision 230Th and 232Th in the Norwegian Sea and Denmark by thermal ionization mass spectrometry

Seawater samples (1–2 liters) were collected from the Norwegian Sea and Denmark Strait and analyzed for 230Th and 232Th using highly sensitive thermal ionization mass spectrometry (TIMS). Depth profiles of dissolved 230Th and 232Th are characterized by surface water minima (&lt;1 fg/kg, &lt;5 pg/kg), subsurface maxima (12 fg/kg, 134 pg/kg), and intermediate concentrations that progressively decrease toward the bottom (∼5 fg/kg, ∼17 pg/kg), respectively. The lack of an increase in 230Th with depth is suggested to result from the short ventilation age of Norwegian Sea Deep Water combined with enhanced scavenging near the basin margins. The 230Th maximum is attributed to advection of high 230Th in the Arctic Intermediate Water, whereas the 232Th maximum may be related to a particulate source. The low dissolved 230Th and 232Th concentrations observed in the NADW formation regions implies a minor advective export of these long‐lived Th tracers to the North Atlantic.

Chlorofluorocarbon‐113 in the northeast Atlantic

An automated gas Chromatographic technique to measure the concentrations of chlorofluorocarbon 113 (CFC‐113:CCl2FCClF2) dissolved in seawater has been developed. The method also quantifies chlorofluorocarbons 11 and 12 (CFC‐11:CCl3F and CFC‐12:CCl2F2). Seawater collected from Niskin bottles in ground‐glass syringes is stripped by a gas stream and concentrated on a cryogenic trap in the manner of Bullister and Weiss (1988) and Gammon et al. (1982). By isolating and heating the trap, the chlorofluorocarbon compounds are reliberated and injected onto a high‐resolution capillary gas Chromatographie column, followed by electron‐capture detection. The analysis time for each sample is less than 15 min. Surface seawater precisions are 2.9%, 2.4%, and 1.2% for CFC‐113, CFC‐11, and CFC‐12, with detection limits of 0.003–0.004, 0.02, and 0.03‐0.05 pmol L−1, respectively. Although these statistics do not compare favorably with other CFC‐11 and CFC‐12 techniques (precision ∼1%, detection limit ∼0.005 pmol L−1 (Bullister and Weiss, 1988)), the dynamic ranges of the CFC‐113:CFC‐11 and CFC‐113:CFC‐12 “ventilation ages” are 20:1, better than that of the best CFC‐11:CFC‐12 age, albeit with inferior precisions. Estimates of the solubility ratios of CFC‐113:CFC‐11 and CFC‐113:CFC‐12 are 0.303 and 1.22, disagreeing with the work of Wisegarver and Gammon (1988), whose CFC‐113 results are believed to be boosted by coelution with methyl bromide. The optimum tracer ventilation age resolution is ±0.9 years for both CFC‐113:CFC‐11 and CFC‐113:CFC‐12 from a cast considered in the northeastern Atlantic. A plot of CFC‐113:CFC‐12 ventilation age is presented on an outcropping isopycnal. A strong correlation with pressure and dissolved oxygen concentration is noted and an oxygen utilization rate between 4.2 and 5.5±0.4 μmol L−1 yr−1 is implied, depending on the choice of CFC‐113 atmospheric history.

Tracer dating and ocean ventilation

The interpretation of transient tracer observations depends on difficult to obtain information on the evolution in time of the tracer boundary conditions and interior distributions. Recent studies have attempted to circumvent this problem by making use of a derived quantity, age, based on the simultaneous distribution of two complementary tracers, such as tritium and its daughter, helium 3. The age is defined with reference to the surface such that the boundary condition takes on a constant value of zero. We use a two‐dimensional model to explore the circumstances under which such a combination of conservation equations for two complementary tracers can lead to a cancellation of the time derivative terms. An interesting aspect of this approach is that mixing can serve as a source or sink of tracer based age. We define an idealized “ventilation age tracer” that is conservative with respect to mixing, and we explore how its behavior compares with that of the tracer‐based ages over a range of advective and diffusive parameters.