The water column distributions of thorium isotopes in the western North Pacific

Four vertical profiles of²³⁰Th and²²⁸Th were determined using large volume water samples in the western North Pacific. An almost linear increase of²³⁰Th with depth was observed for all of the profiles for which the unidirectional first order scavenging model was difficult to explain. We developed a model which included a dissolved-particulate transformation as well as parameters of the scavenging model. Application of the model to the vertical distributions of total and the GEOSECS particulate Th isotopes (²³⁰Th and²³⁴Th) yielded the residence time of dissolved Th with respect to adsorption to particles and the turnover time of particulate Th to be 235 days and 57 days, respectively. The Th isotopes appeared to be carried down the water column by fine particles with a mean settling velocity of 1 m/day which continually release Th into sea water as well as pick up Th from the water along their journey to the bottom.For²²⁸Th, a large excess over²³²Th was observed throughout the water column with pronounced high concentrations in surface and bottom waters, suggesting that the²²⁸Th was derived from²²⁸Ra diffused out of sediments. The vertical distributions of²²⁸Th seemed to be significantly influenced by lateral mixing along isopycnals.     

Natural radionuclides in the water of Narragansett Bay

The concentrations of radionuclides of the U-Th series (²³⁸U,²³⁴Th,²³⁴U,²³⁰Th,²²⁶Ra,²¹⁰Pb,²¹⁰Po, and²³²Th,²²⁸Ra,²²⁸Th) in the water of Narragansett Bay are reported. Analysis of the total, particulate, dissolved and colloidal forms of Th isotopes reveal a consistent removal behavior which is controlled mainly by the particulate matter concentration and the sediment resuspension rate. Half-removal times of Th from solution onto particles range from 1.5 to 15 days, and settling velocities of Th containing particles range generally between 1 and 11 m/day.²¹⁰Pb and²¹⁰Po concentrations are seasonally dependent, with higher concentrations and slower removal during the early summer (half-removal times from solution onto particles of 1–5 days in winter and up to 2 months in early summer).     

Natural and anthropogenic radionuclide distributions in the northwest Atlantic Ocean

We have used in-situ pumps which filter large volumes of sea water through a 1 μm cartridge prefilter and two MnO₂-coated cartridges to obtain information on dissolved and particulate radionuclide distributions in the oceans. Two sites in the northwest Atlantic show subsurface maxima of the fallout radionuclides¹³⁷Cs,^239,240Pu and²⁴¹Am. Although the processes of scavenging onto sinking particles and release at depth may contribute to the tracer distributions, comparison of predicted and measured water column inventories suggests that at least 35–50% of the Pu and²⁴¹Am are supplied to the deep water by advection.The depth distributions of the naturally occurring radionuclides²³²Th,²²⁸Th and²³⁰Th reflect their sources to the oceans.²³²Th shows high dissolved concentrations in surface waters, presumably as a result of atmospheric or riverine supply. Activities of²³²Th decrease with depth to values 0.01 dpm/1000 l.²²⁸Th shows high activities in near surface and near bottom water, due to the distribution of its parent,²²⁸Ra. Dissolved²³⁰Th, produced throughout the water column from²³⁴U decay, increases with depth to 3000 m. Values in the deep water (> 3000 m) are nearly constant ( 0.6–0.7 dpm/1000 l), and the distribution of this tracer (and perhaps other long-lived particle-reactive tracers as well) may be affected by the advection inferred from Pu and²⁴¹Am data.The ratio of particulate to dissolved activity for both²³⁰Th and²²⁸Th is 0.15–0.20. This similarity precludes the calculation of sorption rate constants using a simple model of reversible sorption equilibrium. Moreover, in mid-depths²²⁸Th tends to have a higher particulate/dissolved ratio than²³⁰Th, suggesting uptake and release of²³⁰Th and²²⁸Th by different processes. This could occur if²²⁸Th, produced in surface water, were incorporated into biogenic particles formed there and released as those particles dissolved or decomposed during sinking.²³⁰Th, produced throughout the water column, may more closely approach a sorption equilibrium at all depths.²³⁰Th,²⁴¹Am and^239,240Pu are partitioned onto particles in the sequence Th > Am > Pu with 15% of the²³⁰Th on particles compared with 7% for Am and 1% for Pu. Distribution coefficients (K_d) are 1.3–1.6 × 10⁷ for Th, 5–6 × 10⁶ for Am and 7–10 × 10⁵ for Pu. The lower reactivity for Pu is consistent with analyses of Pu oxidation states which show 85% oxidized (V + VI) Pu. However, theK_d value for Pu may be an upper limit because Pu, like²²⁸Th, may be incorporated into particles in surface waters and released at depth only by destruction of the carrier phase.     

The removal rates of234Th and228Th from waters of the New York Bight

The mean residence time (τ) of thorium with respect to non-radioactive removal from water was determined as a function of location in three traverses of the New York Bight using both²³⁴Th/²³⁸U and²²⁸Th/²²⁸Ra. τ correlates well with salinity increasing from about ten days near shore to 70 days at the shelf break. It shows a much weaker correlation with suspended matter concentrations both because suspended matter varies in its scavenging efficiency and because salinity is a longer-term integrator of scavenging potential.τ's computed from²²⁸Th/²²⁸Ra were often higher than those computed from²³⁴Th/²³⁸U either because of the detrital matter present as reflected in the²³²Th concentrations or because the water parcels had recently arrived from environments of lower scavenging efficiency.Anomalous isotopic concentrations found in three adjacent samples can best be explained as the result of an episodic release of²²⁸Ra from bottom sediments at a rate 200 times the normal one.     

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. Natural²²⁶Ra and²³⁴Th are used as yield tracers. In the equatorial Pacific the²²⁸Ra 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 10⁵ and 10⁷ cm²/s. The perturbing effects of advection and equatorial upwelling west of the Galapagos Islands are noted. Off the coast of Southern California a vertical²²⁸Ra distribution gives an apparent diffusivity of 1.6 cm²/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 ratios²²⁸Th/²²⁸Ra and²³⁴Th/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. The²³²Th content of seawater sampled is less than 0.1 μg/1000 1; most of the published values for seawater²³²Th could well be too high. A comparison of the two methods of determining²²⁸Ra (via²²⁸Ac and via²²⁸Th) made on 64 seawater samples shows that the time delay required by the²²⁸Th method is more than compensated by its better analytical simplicity and precision.     

Fluxes of uranium and thorium series isotopes in the Santa Barbara Basin

Samples from the MANOP Santa Barbara Basin sediment trap intercomparison were analyzed for the isotopes of uranium, thorium, radium, lead, and polonium. All of the traps showed approximately the same compositions and isotopic ratios, indicating that they trapped similar materials. The²³⁴Th flux via falling particles was very close to the flux predicted from the production and scavenging rates of²³⁴Th from the water column. The²¹⁰Pb content of the trapped particles and the surface sediments were the same, however, the measured flux of²¹⁰Pb was seven times greater than the predicted flux. Predicted and measured fluxes of²²⁸Th and²¹⁰Po were similarly out of balance. To explain this apparent inconsistency, we suggest (as others have done) that the Santa Barbara Basin is an area where scavenging from the water column is intensified and where sediments deposited initially on the margins may be physically remobilized on a short time scale. These two effects increase the apparent area from which the basin derives the longer-lived isotopes but does not increase significantly the supply of the short-lived²³⁴Th.     

The residence time of thorium in surface sea water and its implications regarding the rate of reactive pollutants

The activity ratio of²²⁸Th/²²⁸Ra in the open surface ocean averages 0.21. This suggests that thorium is removed from surface water in about 0.7 yr. As plant matter is cycled within the surface sea on a similar time scale, the suggestion is made that highly reactive compounds are inadvertently removed by plants in their quest for the critical nutrients nitrate and phosphate. Combined with the coefficient for horizontal eddy diffusivity obtained from the distribution of²²⁸Ra in surface sea water this result provides a basis for the prediction of the distribution of “reactive” pollutants released to the surface sea from coastal areas.     

Investigations of gram quantities of Atlantic and Pacific surface particulates

Particulates amounting to 0.1–2.0 g efficiently collected from large volumes of Atlantic and Pacific surface waters have been analyzed for carbonate, opal, quartz and several natural and man-made radioisotopes.The concentrations of particles range between 10 and 600 μg/kg. In the equatorial regions particle concentrations are low and similar in both the oceans. At higher latitudes (>30°N or S), the Atlantic waters, however, have higher concentrations of particles compared to those in the Pacific. The latitudinal distribution exhibits a north-south symmetry with higher concentrations in the 30°–60° belt. Based on the particulate abundance for CaCO₃ and opal and their sedimentation, we have estimated their production and in-situ integrated dissolution rates for a few regions.Radioisotopes having different source functions, namely¹⁴C and²³⁹Pu injected due to nuclear weapon tests,²³⁴Th,²³⁰Th and²²⁸Th produced in-situ in seawater,²³²Th which derives primarily from land,²¹⁰Pb introduced via wet precipitations and²²⁶Ra introduced through diffusion from deep-sea sediments have been measured in the particulates. The relative enrichment factors for these nuclides in particles vary as Th ? Pu > Pb > Ra. The atmospheric bomb fallout pattern is discernible in the surface particulates; the²³⁹Pu concentration increases with latitude in both the hemispheres; however, the values are about a factor of two lower in the southern hemisphere.The distribution pattern of radioisotopes is found to be complex, even for²³⁴Th whose source function in the oceans is uniform. In view of the differences in the source functions it becomes possible to delineate the principal geochemical/geophysical processes which determine the concentrations of these nuclides in surface waters.     

Chemical and radiochemical investigations of surface and deep particles of the Indian Ocean

The distribution of “ash” (the non-combustible fraction of marine suspended matter) and concentrations of particulate Al, Ca, Fe, Cr, Ni, Cu, Sr and²³⁴Th in surface waters and of²¹⁰Pb,²³⁰Th and²³⁴Th in two vertical profiles (385–4400 m) of the Indian Ocean are reported.The ash concentrations in surface waters follow the primary productivity pattern, with higher abundances in samples south of 40°S and lower concentrations in the equatorial and subtropical regions. Opaline silica and CaCO₃ are the dominant components of the ash in samples from >40°S and from 7°N to 39°S, respectively. Aluminosilicates are only a minor constituent of the surface particulate matter. The metal/Al ratios in the surface particles are significantly higher compared to their corresponding crustal ratios for all the metals analyzed in this work. Comparison of enrichment factors between marine aerosols, plankton and surface oceanic particles, seem to indicate that this high metal/Al ratio in surface particles most likely arises from their involvement in marine biogeochemical cycles. Particulate²³⁴Th activity in surface waters parallels the ash abundance implying that its scavenging efficiency from surface waters depends on the particulate concentration.The particulate²³⁰Th and²¹⁰Pb concentration profiles increase monotonously with depth. It is difficult to ascribe this increase to a process other than the in-situ vertical scavenging of²³⁰Th and²¹⁰Pb from the water column by settling particles. The mean settling velocities of particles calculated from the particulate²³⁰Th data using a one-dimensional settling model is about 2 × 10^?3 cm/s. The settling velocity computed from the particulate²³⁰Th profiles does not appear to be compatible with the particulate²¹⁰Pb depth profiles; one possible explanation to account for the disparity would be that²³⁰Th and²¹⁰Pb are scavenged by different size populations of particles.On the whole, the geographic distribution of particulate matter, their composition and settling velocities in the Atlantic, Pacific and Indian Oceans are similar indicating that they are controlled by quite similar processes in the marine hydrosphere.     

The thorium isotope content of ocean water

²³²Th concentrations of surface and deep Pacific Ocean waters are 0.01–0.02 dpm/1000 kg (60 pgm/kg). The²³⁰Th activity is 0.03–0.13 dpm/1000 kg in surface waters and 0.3–2.7 dpm/1000 kg in deep waters. Chemical residence times based on in situ production from parent isotopes are about the same for²³⁰Th and²²⁸Th in surface waters (1–5 years) but are ten times greater for²³⁰Th in deep waters (10–100 years). Apparently there are additional sources of²³⁰Th into deep waters. At MANOP site S manganese nodule tops are enriched in Th isotopes by adsorption of Th from seawater and not by incorporation of Th-rich particulates.     

Uranium and thorium isotopes in the rivers of the Amazonian basin: hydrology and weathering processes

Two expeditions (October 1989 and May 1992) were carried out to two points of the main Amazon River channel and four tributaries. The Solimões and Madeira rivers, taking their origin in the Andes, are whitewater rivers. The Negro River is a typical acid, blackwater river. The Trombetas River flows through bauxite‐rich areas, and is characterized by low concentrations of dissolved humic substances. The ²³⁸U, ²³⁴U, ²³²Th and ²³⁰Th activities were recorded from dissolved, suspended particulate phases and river bank sediments. The latter were analysed for their ²²⁶Ra, ²²⁸Ra and ²¹⁰Pb contents, and also subjected to leaching with 0·2 M hydroxylamine–hydrochloride solution to determine the concentrations of radionuclides bound to amorphous Fe hydroxides and Mn oxides and hydroxides. The dissolved U average concentration in the Amazon system is ten times lower than the mean world river concentration. The uranium concentration observed at Óbidos in the lower Amazon (0·095 µg L^?1), where the U content in the river bank sediments and suspended matter is lowest, suggests U release from the solid phase during river transport. About 485 t of U are transported annually to the Amazon delta area in dissolved form, and 1943 t bound to suspended particulate matter. Total U and Th concentrations in the river bank sediments ranged from 1·59 to 7·14 µg g^?1 and from 6·74 to 32 µg g^?1, respectively. The highest concentrations were observed in the Trombetas River. The proportion extracted by means of the hydroxylamine solution (HL) was relatively high for U in the Trombetas river bank sediment (31%) and for Th in the Solimões sediment (30%). According to the alpha recoil effects, the ²³⁴U/²³⁸U activity ratios of the Andean river waters and downstream Amazon water (Óbidos) were >1, but were <1 in the Negro River (at Manaus). The activity ratios of dissolved U correlate with pH and also with the U activity ratios in the river bank sediment hydroxylamine extracts. As expected, the ²³⁴U/²³⁸U activity ratios in river bank sediments were <1 in the Andean rivers and in the downstream Amazon, but they were >1 in the Trombetas and Negro rivers. Such ratios probably result from the binding of dissolved uranium to solid sediment. The ²²⁸Th/²³²Th ratios of river bank sediments were close to unity (except for the Negro River, where it is lower), suggesting no significant Th exchanges between the river water and the sediment. The ²²⁶Ra/²³²Th activity ratios were <1, and the ²²⁶Ra/²²⁸Ra activity ratios generally were significantly higher than the activity ratios of their respective parents. This perhaps is the result of easier leaching of the ²²⁶Ra parent, ²³⁰Th, from solid material (owing to the alpha recoil effect) than of the ²²⁸Ra parent. Uranium and thorium isotopes were used as tools to evaluate the chemical weathering rate of rocks in the Amazon system, which was estimated to be 2·7 cm 1000 year^?1 s^?1. Copyright © 2002 John Wiley & Sons, Ltd.     

Thorium isotopes （22STh, 230Th, 232Th） and applications in reconstructing the Yangtze and Yellow River floods

In the past decades, the floods of the Yangtze and Yellow River introduced unexpected changes of the ecological community and sedimentary dynamics in the East China Sea （ECS）. To reconstruct the flood events in the ECS, 228Th, 230Th and 232Th have been examined in a sediment core. The specific activities of three thorium isotopes have good positive relations with fine fractions （〈63 μm）, indicating that Th activity concentrations heavily depend upon the sediment grain size. The size-normalized activities of 228Th, 23-Th and 232Th showed significant variations. Coincidences between the higher Th activities and historical floods of the Yangtze and Yellow River demonstrated that size-normalized Th recorded the two rivers＇ flood events. The activity ratios of thorium isotopes, i.e. 230Th/232Th and 228Th/232Th, also showed similar patterns to the historical river floods. In three periods （1740s, 1840-1860s and 1930-1960s）, characterized by frequent floods, the thorium activity ratios were fairly low and close to the Yangtze and Yellow River estuary sediments, coinciding with the less oceanic 228Th and 230Th contributions during the flooding periods. Accordingly, these results support the size-normalized Th activity and thorium ratios as proxies of the river floods in coastal seas.     

Removal of234Th from a coastal sea: Funka Bay,Japan

In Funka Bay of Hokkaido, Japan, seawater, suspended matter and settling matter were collected once every month in the summer of 1974. These samples were analyzed for²³⁴Th, a short-lived daughter of dissolved²³⁸U. A pronounced disequilibrium between²³⁴Th and²³⁸U, and a highly variable concentration of²³⁴Th were found. Positive correlation, however, exist among the deficiency of²³⁴Th relative to²³⁸U in seawater, the concentration of particulate²³⁴Th, the fraction of particulate²³⁴Th to total²³⁴Th in seawater, the total dry weight of suspended matter, and the primary productivity during the month previous to sampling. The specific activity of²³⁴Th for the settling particles (620 ± 170 dpm/g) was nearly equal to that for suspended particles (720 ± 600 dpm/g) but much greater than that for plankton (47 ± 24 dpm/g). These facts suggest that suspended particles are somehow closely related to the removal of heavy metals from seawater, in spite of the negligibly small settling flux of suspended matter. The residence time of thorium in Funka Bay (mean depth: 60 m) is found to be about 60 days, which is nearly equal to those of²¹⁰Pb and²¹⁰Po.     

The effects of alteration and the interpretation of heat flow and radioactivity data—a reply to R.U.M. Rao

The first²²⁴Ra (t_1/2 = 3.64days) measurements from mixing zones of estuarine systems are presented for the Pee Dee River-Winyah Bay and Delaware Bay Estuaries. High-resolution gamma-ray spectrometry was used to determine²²⁴Ra,²²⁸Ra (t_1/2 = 5.7years), and²²⁶Ra (t_1/2 = 1622years) activity ratios. Desorption and diffusion from suspended and bottom sediments contributes to the non-conservative increases of the three isotopes in each systems. In Delaware Bay²²⁴Ra concentrations were nearly constant over the 2.5‰ to 15‰ salinity range where two turbidity maximum zones are located.²²⁸Th scavenging onto the suspended particles in the turbid zones may supply a regenerative source of²²⁴Ra in this system. Samples collected on the ebb and flood tide from a salt marsh along Delaware Bay have a 5-fold increase in²²⁴Ra from flood to ebb and 3- and 2-fold increases for²²⁸Ra and²²⁶Ra respectively, indicating salt marshes are another source of radium to estuarine waters.     

Sediment focusing creates 100-ka cycles in interplanetary dust accumulation on the Ontong Java Plateau

The accumulation of extraterrestrial ³He, a tracer for interplanetary dust particles (IDPs), in sediments from the Ontong Java Plateau (OJP; western equatorial Pacific Ocean) has been shown previously to exhibit a regular cyclicity during the late Pleistocene, with a period of ∼100 ka. Those results have been interpreted to reflect periodic variability in the global accretion of IDPs that, in turn, has been linked to changes in the inclination of Earth’s orbit with respect to the invariable plane of the solar system. Here we show that the accumulation in OJP sediments of authigenic ²³⁰Th, produced by radioactive decay of ²³⁴U in seawater, exhibits a 100-ka cyclicity similar in phase and amplitude to that evident in the ³He record. We interpret the similar patterns of ²³⁰Th and ³He accumulation to reflect a common origin within the ocean-climate system. Comparing spatial and temporal patterns of sediment accumulation against regional patterns of biological productivity and against the well-established pattern of CaCO₃ dissolution in the deep Pacific Ocean leads to the further conclusion that a common 100-ka cycle in accumulation of biogenic, authigenic and extraterrestrial constituents in OJP sediments reflects the influence of climate-related changes in sediment focusing, rather than changes in the rate of production or supply of sedimentary constituents.     

The uranium and thorium decay series nuclides in Mt. St. Helens effusives

The concentrations of the radionuclides²³⁸U,²³⁰Th,²²⁶Ra,²¹⁰Pb,²¹⁰Po,²³²Th,²²⁸Ra and²²⁸Th and the abundances of major elements were determined in samples from all major eruptions of Mt. St. Helens from May 18, 1980 through June 21, 1981. During this time the effusives changed from plagioclase-phyric dacite to a more andesitic composition but the concentrations of U and Th series nuclides were measurably invariant. The average²³²Th/²³⁸U weight ratio in the rocks is 2.4 and the²³⁰Th/²³²Th activity ratio equals the²³⁸U/²³²Th activity ratio indicating no fractionation of U from Th during magma genesis.²²⁶Ra activity is in excess (～40% on average) of its parent²³⁰Th whereas²²⁸Ra is in radioactive equilibrium with its parent²³²Th, constraining the time of magma formation between 30 and 10⁴ years prior to eruption. The²¹⁰Pb/²²⁶Ra activity ratios in the samples average 1.0, with a 20% scatter on either side, but allowing for volatile²¹⁰Pb loss at time of eruption excess²¹⁰Pb over²²⁶Ra is inferred, indicating that the time of magma formation was within the last 150 years.²¹⁰Po was virtually absent in the samples immediately after eruption, indicating its total loss by volatilization during eruption. The quantity of²¹⁰Po volatilized during the May 18, 1980 event is estimated to be in the range of 300 Ci from the effusives and as much as 5000 Ci total including losses from heated slide material. The²²²Rn activity volatilized should have been comparable to the²¹⁰Po activity released.     

Ra in the Dead Sea

The²²⁸Ra concentrations of the Dead Sea waters range from 0.13 to 1.48 dpm kg⁻¹, two to three orders of magnitude higher than those of ocean waters and lake waters. However, the²²⁸Ra/²²⁶Ra activity ratios, (0.12–1.29) × 10⁻², are in the range reported for the hydrosphere.The surface waters of the Dead Sea are enriched in²²⁸Ra by a factor of about three over the near-bottom waters. There is a factor of about two spatial variability in the mid-depth Ra concentrations at the two profile stations. The near-bottom²²⁸Ra gradients yield vertical eddy diffusivity coefficient (K) of 2.0 and 0.4 cm² s⁻¹ at profile locations 1 and 2 respectively. These values are comparable to those measured in oceans and lakes.     

Interpretation of the 231Pa/230Th paleocirculation proxy: New water-column measurements from the southwest Indian Ocean

Measurements of ²³¹Pa, ²³⁰Th and ²³²Th concentrations have been made on five water-column profiles along the western margin of the Madagascar and Mascarene Basins in the southern Indian Ocean. These measurements help to fill a significant gap in the global coverage of water-column ²³²Th, ²³⁰Th and ²³¹Pa data. ²³²Th concentrations vary, but generally increase with depth, suggesting higher particle loading in deeper waters, and the presence of a significant dissolved fraction of ²³²Th. ²³⁰Th concentrations increase with depth, and profiles are similar to the average of existing data from other regions. ²³¹Pa concentrations, on the other hand, show significant depth structure, apparently reflecting the various water masses sampled at this location. The modified remnants of North Atlantic Deep Water are found at a depth of ≈ 2000 m and exhibit elevated ²³¹Pa concentrations exported from the South Atlantic. Antarctic Intermediate and Bottom Waters have lower ²³¹Pa, probably due to scavenging onto opal particles during transit from the Southern Ocean. The differences between water masses raises a question: which water mass is important in controlling the ²³¹Pa/²³⁰Th ratio in underlying sediments? A simple one-dimensional model is used to demonstrate that the ²³⁰Th and ²³¹Pa exported to sea-floor sediments last equilibrates with waters close to the seafloor (within ≈ 1000 m), rather than averaging the whole water column. These findings suggest that ²³¹Pa_xs/²³⁰Th_xs in sediments provides information primarily about deep-water masses. In this region, sedimentary records will therefore provide information about the past flow of Antarctic Bottom Water into the Indian Ocean. Interpretation of data from other regions, such as the North Atlantic where this proxy has most successfully been applied, requires careful consideration of regional oceanography and knowledge of the composition of the water masses being investigated.     

The natural radiochemical and growth records in contemporary hermatypic corals from the Atlantic and Caribbean

The skeletal banding observed in vertical cross-sections of two species of coral,Montastrea annularis from Jamaica andDiploria labrynthiformis from Bermuda, has been confirmed to be annual by ingrowth of²²⁸Th towards radioactive equilibrium with²²⁸Ra taken up from seawater. In addition the Bermuda coral is shown to have incorporated²¹⁰Pb at essentially constant specific activity over at least the last thirty years.The individual annual patterns of growth banding in a suite of specimens ofD. Labrynthiformis from Bermuda are shown to be correlative. The mass of CaCO₃ accumulated each year is positively correlated with the band width for the corresponding year.