Coral Radiocarbon Research Articles

Radiocarbon evidence for the stability of polar ocean overturning during the Holocene

Proxy-based studies have linked the pre-industrial atmospheric p_{{rm{CO}}_{2}} rise of ∼20 ppmv in the mid- to late Holocene to an inferred increase in the Southern Ocean overturning and associated biogeochemical changes. However, the history of polar ocean overturning and ventilation through the Holocene remains poorly constrained, leaving important gaps in the assessment of the feedbacks between changes in ocean circulation and the carbon cycle in a warm climate state. The deep-ocean radiocarbon content, which provides a measure of ventilation, responds to circulation changes on centennial to millennial time scales. Here we present absolutely dated deep-sea coral radiocarbon records from the Drake Passage, between South America and Antarctica, and Reykjanes Ridge, south of Iceland, over the Holocene. Our data suggest that ventilation in the Antarctic circumpolar waters and North Atlantic Deep Water is surprisingly invariant within proxy uncertainties at our sampling resolution. Our findings indicate that long-term, large-scale polar ocean overturning has not been disturbed to a level resolvable by radiocarbon and is probably not responsible for the millennial atmosphere p_{{rm{CO}}_{2}} evolution through the Holocene. Instead, continuous nutrient and carbon redistribution within the water column following deglaciation, as well as changes in land organic carbon stock, might have regulated atmospheric CO2 budget during this period.

Plutonium isotopes in the North Western Pacific sediments coupled with radiocarbon in corals recording precise timing of the Anthropocene

Plutonium (Pu) has been used as a mid-twentieth century time-marker in various geological archives as a result of atmospheric nuclear tests mainly conducted in 1950s. Advancement of analytical techniques allows us to measure 239Pu and 240Pu more accurately and can thereby reconstruct the Pacific Pu signal that originated from the former Pacific Proving Grounds (PPG) in the Marshall Islands. Here, we propose a novel method that couples annual banded reef building corals and nearshore anoxic marine sediments to provide a marker to precisely determine the start of the nuclear era which is known as a part of the Anthropocene. We demonstrate the efficacy of the methods using sediment obtained from Beppu Bay, Japan, and a coral from Ishigaki Island, Japan. The sedimentary records show a clear Pu increase from 1950, peaking during the 1960s, and then showing a sharp decline during the 1970s. However, a constantly higher isotope ratio between 239Pu and 240Pu suggest an additional contribution other than global fallout via ocean currents. Furthermore, single elevations in 240Pu/239Pu provide supportive evidence of close-in-fallout similar to previous studies. Coral skeletal radiocarbon displays a clear timing with the signatures supporting the reliability of the Beppu Bay sediments as archives and demonstrates the strength of this method to capture potential Anthropocene signatures.

Air-sea CO2 exchange rate in the northern Indian Ocean based on coral radiocarbon records

Corals growing in shallow oceanic regions record the radiocarbon variations in ocean surface waters. Tracing bomb radiocarbon in various carbon reservoirs, can help understand the pathways and rates of carbon exchange between the different reservoirs. The northern Indian Ocean comprising two adjoining basins the Arabian Sea and the Bay of Bengal demonstrates contrasting behaviour both in its hydrological condition and CO2 sequestration. In this study, coral based bomb radiocarbon records from the Lakshadweep Islands and the Andaman Islands in the northern Indian Ocean have been analysed. The estimates of air-sea CO2 exchange rate based on bomb radiocarbon for the Lakshadweep is 13.4 ± 2.1 mol m−2 yr−1 and for the northern Andaman is 8.8 ± 1.3 mol m−2 yr−1. The bomb radiocarbon based air-sea CO2 exchange over the Indian Ocean follow the trend of empirical relationship with wind speed. Using the air-sea CO2 exchange rates, the net regional CO2 fluxes over the Lakshadweep and the northern Andaman region has been determined.

Multidecadal oceanographic changes in the western Pacific detected through high‐resolution bomb‐derived radiocarbon measurements on corals

AbstractHigh‐resolution measurements of radiocarbon (14C) in corals can be used to reconstruct past variability in ocean conditions. Here we report seasonal Δ14C changes in coral from Ishigaki Island, Japan, and compare with previously reported data from Palau and Guam. Our data clearly indicate a significant increase in Δ14C from 1947 to 1998 related to atmospheric nuclear bomb testing. The three early Δ14C spikes related to the atmospheric nuclear bomb tests in the US Proving Grounds at Bikini and Enewerak atoll conducted in 1954, 1956, and 1958 were detected from the Ishigaki coral. After 1976, variability in the Mindanao Dome region related to North Equatorial Current (NEC) bifurcation latitude migration affected the Δ14C difference between Palau and Guam, whereas the difference between Ishigaki and Guam was not correlated with the bifurcation latitude. The Δ14C difference between Ishigaki and Guam may be due to mesoscale eddies in the Kuroshio area. On the decadal scale, the northward shift of NEC bifurcation latitude after 1976, the year as known as Pacific Decadal Oscillation regime shift from negative to positive, was concurrent with the abundant westward‐propagating mesoscale eddies in the Subtropical Countercurrent region and stronger Kuroshio transport off the east Taiwan, which may be represented by a smaller Δ14C difference between Ishigaki and Guam after 1976.

Eastern tropical North Pacific coral radiocarbon reveals North Pacific Gyre Oscillation (NPGO) variability

Fluctuations in oceanic circulation and upwelling associated with the North Pacific Gyre Oscillation (NPGO) are the largest source of salinity and nutrient concentration variability across the Pacific basin. Recent observations suggest NPGO-like variability is intensifying, but longer, “pre-instrumental” records are required to improve our understanding of NPGO amplitude and phase change. Here, using measurements of coral skeletal chemistry from San Benedicto Island in the Eastern Tropical North Pacific (ETNP), we assess this region's suitability for reconstructing NPGO behavior. We find that coral geochemical proxy measurements of ETNP salinity and dissolved inorganic carbon radiocarbon (Δ14C) content reflect NPGO-driven gyre circulation and regional coastal upwelling. These results provide the basis for reconstructing NPGO-related ocean conditions hundreds of years prior to the modern observational record.

Upwelling of Pacific Intermediate Water in the South China Sea Revealed by Coral Radiocarbon Record

AbstractAnnual radiocarbon from a massive Porites lutea coral collected from Hon Tre Island, Vietnam, South China Sea (SCS) was analyzed over a ~100-yr-long period from AD 1900 to 1986. The pre-bomb results from 1900–1953 show a steady Δ14C value of –54.4±1.8‰ (n=60). These values are similar to coral records located in the central and southern SCS and from Indonesian waters, but are lower than those from Japan. Following the input of anthropogenic bomb 14C, our results show a sharp increase in Δ14C from 1960, reaching a peak value of 155.3‰ in 1973. The Hon Tre Island post-bomb Δ14C values are lower than those of other corals located in the SCS and Japan, but higher compared to those in the Indonesian Seas. This study infers a seasonal input of upwelled water depleted in 14C from the deeper SCS basin that originates from the tropical Pacific via the Luzon Strait. The bifurcation of the North Equatorial Current feeds the surface and intermediate currents in the SCS and Makassar Strait region. However, unlike the Makassar site, this study’s coral Δ14C does not receive lower 14C water from the South Pacific Equatorial Current. The Vietnam record therefore represents a unique oceanographic position, reflecting the seasonal influence of older, deeper SCS waters that upwell periodically in this area and have modified the surface waters locally in this region over the last 100 yr.

Ventilation time scales of the North Atlantic subtropical cell revealed by coral radiocarbon from the Cape Verde Islands

AbstractWe present coral‐ and sclerosponge‐based reconstructions of the 14C content in North Atlantic dissolved inorganic carbon (DIC) during the last ~100 years from the subtropical cells (STCs). These waters are sensitive to the dynamics of the shallow overturning meridional circulation that transports heat and water masses from the subtropics to the tropics. We use these records to investigate the circulation patterns of the off‐equatorial upwelling regions of the STCs, which are not well understood. Coral and sclerosponge skeletons provide long time series of ocean DIC 14C content, a tracer of oceanic circulation, effectively extending the observational record back in time. Sclerosponge data from the Bahamas were used to extend the existing subtropical 14C time series to the 21st century. Coral 14C data from the Cape Verde Islands (1890–2002) captured the 14C signature of water brought to the surface in the off‐equatorial regions of the STC present near the West African coast. We observe a unique postbomb trend at Cape Verde that is similar to the upwelling regions in the Pacific, and we interpret this trend as the result of the slow penetration of bomb 14C into the interior ocean as part of the STC circulation. Using a multibox mixing model we constrain the time history of bomb 14C in the eastern tropical Atlantic, and we estimate a 20 year time scale for ventilation of the thermocline in this area of the ocean. The similarity between the Atlantic and Pacific 14C‐based records of upwelling suggests that both are caused by bomb 14C penetration rather than more complex explanations that invoke changes in thermocline depth (e.g., related to El Niño–Southern Oscillation variability) or changes in the strength of the subtropical cells. Our results offer constraints for models of tropical ocean circulation and anthropogenic CO2 uptake that attempt to reproduce the characteristics of the shallow wind‐driven circulation in the Atlantic.

Growth rates and ages of deep-sea corals impacted by the Deepwater Horizon oil spill

The impact of the April 2010 Deepwater Horizon (DWH) spill on deep-sea coral communities in the Gulf of Mexico (GoM) is still under investigation, as is the potential for these communities to recover. Impacts from the spill include observation of corals covered with flocculent material, with bare skeleton, excessive mucous production, sloughing tissue, and subsequent colonization of damaged areas by hydrozoans. Information on growth rates and life spans of deep-sea corals is important for understanding the vulnerability of these ecosystems to both natural and anthropogenic perturbations, as well as the likely duration of any observed adverse impacts. We report radiocarbon ages and radial and linear growth rates based on octocorals (Paramuricea spp. and Chrysogorgia sp.) collected in 2010 and 2011 from areas of the DWH impact. The oldest coral radiocarbon ages were measured on specimens collected 11km to the SW of the oil spill from the Mississippi Canyon (MC) 344 site: 599 and 55calyr BP, suggesting continuous life spans of over 600 years for Paramuricea biscaya, the dominant coral species in the region. Calculated radial growth rates, between 0.34μmyr−1 and 14.20μmyr−1, are consistent with previously reported proteinaceous corals from the GoM. Anomalously low radiocarbon (Δ14C) values for soft tissue from some corals indicate that these corals were feeding on particulate organic carbon derived from an admixture of modern surface carbon and a low 14C carbon source. Results from this work indicate fossil carbon could contribute 5–10% to the coral soft tissue Δ14C signal within the area of the spill impact. The influence of a low 14C carbon source (e.g., petro-carbon) on the particulate organic carbon pool was observed at all sites within 30km of the spill site, with the exception of MC118, which may have been outside of the dominant northeast–southwest zone of impact. The quantitatively assessed extreme longevity and slow growth rates documented here highlight the vulnerability of these long-lived deep sea coral species to disturbance.

Structural limitations in deriving accurate U-series ages from calcitic cold-water corals contrast with robust coral radiocarbon and Mg/Ca systematics

Radiocarbon and uranium-thorium dating results are presented from a genus of calcitic Antarctic cold-water octocorals (family Coralliidae), which were collected from the Marie Byrd Seamounts in the Amundsen Sea (Pacific sector of the Southern Ocean) and which to date have not been investigated geochemically. The geochronological results are set in context with solution and laser ablation-based element/Ca ratios (Li, B, Mg, Mn, Sr, Ba, U, Th).Octocoral radiocarbon ages on living corals are in excellent agreement with modern ambient deep-water Δ14C, while multiple samples of individual fossil coral specimens yielded reproducible radiocarbon ages. Provided that local radiocarbon reservoir ages can be derived for a given time, fossil Amundsen Sea octocorals should be reliably dateable by means of radiocarbon.In contrast to the encouraging radiocarbon findings, the uranium-series data are more difficult to interpret. The uranium concentration of these calcitic octocorals is an order of magnitude lower than in the aragonitic hexacorals that are conventionally used for geochronological investigations. While modern and Late Holocene octocorals yield initial δ234U in good agreement with modern seawater, our results reveal preferential inward diffusion of dissolved alpha-recoiled 234U and its impact on fossil coral δ234U. Besides alpha-recoil related 234U diffusion, high-resolution sampling of two fossil octocorals further demonstrates that diagenetic uranium mobility has offset apparent coral U-series ages. Combined with the preferential alpha-recoil 234U diffusion, this process has prevented fossil octocorals from preserving a closed system U-series calendar age for longer than a few thousand years. Moreover, several corals investigated contain significant initial thorium, which cannot be adequately corrected for because of an apparently variable initial 232Th/230Th. Our results demonstrate that calcitic cold-water corals are unsuitable for reliable U-series dating.Mg/Ca ratios within single octocoral specimens are internally strikingly homogeneous, and appear promising in terms of their response to ambient temperature. Magnesium/lithium ratios are significantly higher than usually observed in other deep marine calcifiers and for many of our studied corals are remarkably close to seawater compositions.Although this family of octocorals is unsuitable for glacial deep-water Δ14C reconstructions, our findings highlight some important differences between hexacoral (aragonitic) and octocoral (calcitic) biomineralisation. Calcitic octocorals could still be useful for trace element and some isotopic studies, such as reconstruction of ambient deep water neodymium isotope composition or pH, via boron isotopic measurements.

Radiocarbon and stable isotopes in Palmyra corals during the past century

Annual samples from two Palmyra Atoll corals (Porites lutea) that lived during the past 110years were analyzed for radiocarbon (Δ14C) and δ18O. The Δ14C values decreased 7.6‰ from 1896 to 1953, similar to other coral records from the tropical and subtropical Pacific. Δ14C values rose from ∼−60‰ to ∼+110‰ by 1980 due to the input of bomb radiocarbon from the atmosphere. Elevated Δ14C values were observed for the mid- to late-1950s, suggesting early input of bomb radiocarbon, possibly from the largest Marshall Islands bomb tests in 1954. Secondary aragonite precipitation was identified in a portion of one core using scanning electron microscopy and X-radiography, and was responsible for high δ18O and δ13C values and a correlation between them. The Δ14C results were more resistant to alteration, except when contamination was from the bomb era (>1956).

Disturbances with hiatuses in high-latitude coral reef growth during the Holocene: Correlation with millennial-scale global climate change

Recent studies have reported Holocene millennial-scale climate instability at a global scale. However, the relationship between this climate variability and coral reef growth is still unclear. Field observations and high-precision, in situ coral radiocarbon dating of the excavated trench walls of an uplifted middle-to-late Holocene coral reef on Kodakara Island, located in the pathway of the Kuroshio Current in the northwestern Pacific, show evidence of the existence of disturbances with hiatuses in coral reef growth and coral composition differences before and after the disturbances. We found three disconformities in the reef, and the dating results indicate that disturbances with hiatuses in reef growth occurred at approximately 5.9 to 5.8, 4.4 to 4.0, and 3.3 to 3.2 cal yr B.P. The results also indicate that the second and third events were associated with sea-level oscillation. The timing of the disturbances corresponds well with the periods when the Kuroshio Current was relatively weak and was associated with a relatively cold sea surface temperature, which may have enhanced the cold winter Asian monsoons, and with Holocene North Atlantic ice-rafting cold events. The coral composition clearly changed before and after the disturbances, with gradually reduced diversity resulting in a reef dominated by acroporiid coral. These data led to the hypothesis that coral reef growth was interrupted by suborbital millennial-scale global climate change induced by persistent solar activity during the Holocene in high-latitude coral reefs, such as those in the Northwest Pacific, leading to low diversity in the reefs that experienced each disturbance. Our results may provide new insights into theories of past and future coral reef formation worldwide.

Coral records of central tropical Pacific radiocarbon variability during the last millennium

PALEOCEANOGRAPHY, VOL. 25, PA4212, doi:10.1029/2009PA001788, 2010 Coral records of central tropical Pacific radiocarbon variability during the last millennium Laura K. Zaunbrecher, 1,2 Kim M. Cobb, 1 J. Warren Beck, 3 Christopher D. Charles, 4 Ellen R. M. Druffel, 5 Richard G. Fairbanks, 6 Sheila Griffin, 5 and Hussein R. Sayani 1 Received 12 May 2009; revised 18 May 2010; accepted 7 June 2010; published 10 November 2010. [ 1 ] The relationship between decadal to centennial changes in ocean circulation and climate is difficult to discern using the sparse and discontinuous instrumental record of climate and, as such, represents a large uncertainty in coupled ocean‐atmosphere general circulation models. We present new modern and fossil coral radiocarbon (D 14 C) records from Palmyra (6°N, 162°W) and Christmas (2°N, 157°W) islands to constrain central tropical Pacific ocean circulation changes during the last millennium. Seasonally to annually resolved coral D 14 C measurements from the 10th, 12th–17th, and 20th centuries do not contain significant interannual to decadal‐scale variations, despite large changes in coral d 18 O on these timescales. A centennial‐scale increase in coral radiocarbon from the Medieval Climate Anomaly (∼900–1200 AD) to the Little Ice Age (∼1500–1800) can be largely explained by changes in the atmospheric D 14 C, as determined with a box model of Palmyra mixed layer D 14 C. However, large 12th century depletions in Palmyra coral D 14 C may reflect as much as a 100% increase in upwelling rates and/or a significant decrease in the D 14 C of higher‐ latitude source waters reaching the equatorial Pacific during this time. SEM photos reveal evidence for minor dissolution and addition of secondary aragonite in the fossil corals, but our results suggest that coral D 14 C is only compromised after moderate to severe diagenesis for these relatively young fossil corals. Citation: Zaunbrecher, L. K., K. M. Cobb, J. W. Beck, C. D. Charles, E. R. M. Druffel, R. G. Fairbanks, S. Griffin, and H. R. Sayani (2010), Coral records of central tropical Pacific radiocarbon variability during the last millennium, Paleoceanography, 25, PA4212, doi:10.1029/2009PA001788. 1. Introduction [ 2 ] Ocean circulation changes in the tropical Pacific strongly influence global climate, as demonstrated during El Nino‐Southern Oscillation (ENSO) extremes. During strong El Nino events, a relaxation of the trade winds results in a large reduction of the equatorial upwelling of cooler subsurface waters and reshapes the wind‐driven surface currents in the tropical Pacific [Taft and Kessler, 1991]. This reorganization of equatorial currents causes anomalously warm waters in the eastern and central tropical Pacific, ulti- mately driving a reorganization of the large‐scale global atmospheric circulation. While instrumental data resolve seasonal to interannual variability in tropical Pacific circu- lation [Picaut and Tournier, 1991; Donguy and Meyers, School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia, USA. Now at Department of Geosciences, Georgia State University, Atlanta, Georgia, USA. Physics and Geosciences Department, University of Arizona, Tucson, Arizona, USA. Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California, USA. Earth System Science Department, University of California, Irvine, California, USA. Earth and Planetary Science Department, Rutgers, State University of New Jersey, New Brunswick, New Jersey, USA. Copyright 2010 by the American Geophysical Union. 0883‐8305/10/2009PA001788 1996], the nature of decadal to centennial‐scale changes in tropical Pacific circulation remain unknown. Resolving such low‐frequency ocean circulation variability and its rela- tionship to low‐frequency regional and global climate changes is critical to the improvement of ocean models used for climate prediction. [ 3 ] Radiocarbon ( 14 C) is a useful tracer of water mass mixing, as deep waters that have been isolated from the atmosphere are depleted in 14 C due to radioactive decay, whereas surface waters are relatively enriched. Large sur- face water 14 C gradients arise from horizontal and vertical mixing – upwelling brings relatively depleted 14 C waters to the ocean surface whereas prolonged air‐sea gas exchange in the mid‐ocean gyres drives 14 C enrichment in these areas. Thus, regional water masses are ‘tagged’ with a distinct 14 C signature depending on the regional oceanographic setting. Changes to regional seawater 14 C values through time imply changes in either horizontal or vertical mixing. [ 4 ] Corals are useful tools for the reconstruction of sea- water 14 C signatures as they incorporate the 14 C of the dissolved inorganic carbon of the seawater in which they grow into their skeletal matrix, and can live for decades to centuries [Druffel and Linick, 1978; Dunbar and Cole, 1999; Druffel et al., 2007]. Annual band counting in mod- ern corals and/or U/Th dating in fossil corals ensure accurate, C‐independent absolute chronologies for the construction of coral‐based records of seawater radiocarbon variability through time. Indeed, the incorporation of “bomb 14 C” PA4212 1 of 15

Oceanic climate and circulation changes during the past four centuries from radiocarbon in corals

We analyzed radiocarbon in annual coral bands from the Galápagos Islands in the tropical east Pacific to understand natural variability of past ocean circulation associated with climate change. Variability was observed on El Niño timescales (3–7 years) as well as at decadal and multi‐decadal periods. Low radiocarbon levels persisted for ten years during the early 1600s and for sixteen years during the early 1800s, both coincident with periods of high volcanic aerosol loading in the atmosphere from massive eruptions. Intensification of tropical circulation at this time may be linked to climatically‐controlled changes in the structure of the tropical thermocline that is responsible for delivering cool, upwelled waters to the tropical east Pacific.

Decadal- to interannual-scale source water variations in the Caribbean Sea recorded by Puerto Rican coral radiocarbon

Water that forms the Florida Current, and eventually the Gulf Stream, coalesces in the Caribbean from both subtropical and equatorial sources. The equatorial sources are made up of, in part, South Atlantic water moving northward and compensating for southward flow at depth related to meridional overturning circulation. Subtropical surface water contains relatively high amounts of radiocarbon (14C), whereas equatorial waters are influenced by the upwelling of low 14C water and have relatively low concentrations of 14C. We use a 250 year record of Δ14C in a coral from southwestern Puerto Rico along with previously published coral Δ14C records as tracers of subtropical and equatorial water mixing in the northern Caribbean. Data generated in this study and from other studies indicate that the influence of either of the two water masses can change considerably on interannual to interdecadal time scales. Variability due to ocean dynamics in this region is large relative to variability caused by atmospheric 14C changes, thus masking the Suess effect at this site. A mixing model produced using coral Δ14C illustrates the time varying proportion of equatorial versus subtropical waters in the northern Caribbean between 1963 and 1983. The results of the model are consistent with linkages between multidecadal thermal variability in the North Atlantic and meridional overturning circulation. Ekman transport changes related to tradewind variability are proposed as a possible mechanism to explain the observed switches between relatively low and high Δ14C values in the coral radiocarbon records.

Radiocarbon in corals from the Cocos (Keeling) Islands and implications for Indian Ocean circulation

Annual bands of a Porites coral from the Cocos (Keeling) Islands, eastern Indian Ocean, were analysed by radiocarbon for 1955–1985 AD. A rapid oceanic response of the site to bomb 14C is found, with a maximum Δ14C value of 132‰ in 1975. This value is considerably higher than those for the northwestern Indian Ocean, suggesting that surface waters reaching Cocos are not derived from the Arabian Sea. Instead, Δ14C values for Cocos and those for Watamu (Kenya) agree well over most of the study interval, suggesting that the South Equatorial Current carries 14C‐elevated water rather than 14C‐depleted water westward across the Indian Ocean. This implies that oceanic upwelling in the northwestern Indian Ocean is spatially confined with little contribution to the upper limb of the global thermohaline circulation.

Extracting growth rates from the nonlaminated coralline sponge Astrosclera willeyana using bomb radiocarbon

The isotopic and chemical composition of the skeletons of hermatypic reef building corals have been used for many years to generate proxy time series records of surface oceanic conditions (water temperature and salinity). These types of records have recently been constructed using coralline sponges. Because they are found in a greater range of depths, coralline sponges have the potential to fill in gaps in our understanding of subsurface oceanographic variability. Using coralline sponges together with surface corals, we can obtain a threedimensional (3D) picture of oceanographic/climate variability. However, coralline sponges have one disadvantage compared to hermatypic reef building coral proxies in that most do not have annual density bands and need to be radiometrically dated for age determination. We have measured radiocarbon in 1 mm increments from Astrosclera willeyana sponges collected off the Central and Northern Great Barrier Reef (GBR) and from Truk in the Caroline Islands and compared these radiocarbon profiles to independently dated coral radiocarbon records to examine growth rate variability. Growth rates of the GBR sponges averaged 1.2 ± 0.3 and 1.0 ± 0.3 mm y−1 at north and central sites respectively, but the growth rate can vary by a factor of two over the life span of the sponge. The growth rate of the Truk sponge averaged 1.2 ± 0.1 mm y−1. These growth rates are faster than those measured for other Astrosclera willeyana sponges (0.2–0.7 mm y−1) (Moore et al. 2000; Wörheide 1998).

Intrabasin comparison of surface radiocarbon levels in the Indian Ocean between coral records and three‐dimensional global ocean models

Oceanic uptake and transport of bomb‐produced radiocarbon is used as a diagnostic in global ocean models to test parameterizations of mixing and air‐sea gas exchange between the ocean and atmosphere. A model's ability to simulate bomb‐produced 14C is also a good indicator of its ability to predict uptake of anthropogenic CO2. We have conducted a model‐data comparison of surface radiocarbon time series from coral records from the coasts of Kenya and Sumatra and a suite of dynamical three‐dimensional ocean models that were included in the second phase of the Ocean Carbon‐Cycle Model Intercomparison Project. The coral records comprise the first intrabasin record of surface water Δ14C variability in the equatorial Indian Ocean and provide an independent evaluation of model performance. Differing treatments of lateral subgrid scale mixing in different models appear to be less important than other, unknown factors in explaining differences in results between the models. Those models that include a dynamic vertical mixing scheme appear to be more capable of matching observed coral radiocarbon time series. However, among models with the same parameterization of lateral subgrid scale mixing, there is a large degree of variation, suggesting that at both sites, factors such as resolution, topography, physical forcing and horizontal advection are more important than mixing parameterization in explaining intermodel differences. None of the models reproduce the time lag in the rate of bomb 14C response between the Kenya and Sumatra coral sites. Future efforts are needed to improve model simulation of radiocarbon in surface waters in the equatorial Indian Ocean.

A semiannual radiocarbon record of a modern coral from the Solomon Islands

Radiocarbon records from modern corals have long been recognized for their usefulness as a geochemical tracer of surface ocean waters and oceanic upwelling. Pacific corals are especially interesting because of their potential relevance to El Niño – Southern Oscillation (ENSO) events. At present, the Pacific Ocean is undersampled with respect to radiocarbon time series. This study establishes a 14C time series for a coral from the Solomon Islands, located near the center of the Western Pacific Warm Pool (WPWP). We present radiocarbon results from a Porites colony which grew in Marau Sound, on the east coast of Guadalcanal. A semiannual record of 14C was constructed from measurements of alternating bands which grew continuously from 1944 to 1994. The record reflects the uptake of atmospheric bomb-produced 14C since the late 1950s with superimposed subannual radiocarbon variations, presumably related to changes in ocean circulation. Although the coral radiocarbon is influenced by ENSO events, the record is not closely correlated with the Southern Oscillation Index (SOI) for the same period.

A semiannual radiocarbon record of a modern coral from the Solomon Islands

Radiocarbon records from modern corals have long been recognized for their usefulness as a geochemical tracer of surface ocean waters and oceanic upwelling. Pacific corals are especially interesting because of their potential relevance to El Niño – Southern Oscillation (ENSO) events. At present, the Pacific Ocean is undersampled with respect to radiocarbon time series. This study establishes a 14 C time series for a coral from the Solomon Islands, located near the center of the Western Pacific Warm Pool (WPWP). We present radiocarbon results from a Porites colony which grew in Marau Sound, on the east coast of Guadalcanal. A semiannual record of 14 C was constructed from measurements of alternating bands which grew continuously from 1944 to 1994. The record reflects the uptake of atmospheric bomb-produced 14 C since the late 1950s with superimposed subannual radiocarbon variations, presumably related to changes in ocean circulation. Although the coral radiocarbon is influenced by ENSO events, the record is not closely correlated with the Southern Oscillation Index (SOI) for the same period.

Coral radiocarbon records of Indian Ocean water mass mixing and wind‐induced upwelling along the coast of Sumatra, Indonesia

Radiocarbon (14C) in the skeletal aragonite of annually banded corals track radiocarbon concentrations in dissolved inorganic carbon (DIC) in surface seawater. As a result of nuclear weapons testing in the 1950s, oceanic uptake of excess 14C in the atmosphere has increased the contrast between surface and deep ocean 14C concentrations. We present accelerator mass spectrometric (AMS) measurements of 14C/12C ratios (Δ14C) in Porites corals from the Mentawai Islands, Sumatra (0°S, 98°E) and Watamu, Kenya (3°S, 39°E) to document the temporal and spatial evolution of the 14C zonal gradient in the tropical Indian Ocean. The rise in Δ14C in the Sumatra coral, in response to the maximum in nuclear weapons testing, is delayed by 2–3 years relative to the rise in coral Δ14C from the coast of Kenya. Kenya coral Δ14C values rise quickly because surface waters are in prolonged contact with the atmosphere. In contrast, wind‐induced upwelling and rapid mixing along the coast of Sumatra entrains 14C‐depleted water from the subsurface, which dilutes the effect of the uptake of bomb‐produced 14C by the surface ocean. Bimonthly AMS Δ14C measurements on the Mentawai coral reveal mainly interannual variability with minor seasonal variability. Singular spectrum analysis of the Sumatra coral Δ14C record reveals a significant 3‐year periodicity. These results lend support to the concept that interannual variability in Indian Ocean upwelling and sea surface temperatures is related to ENSO‐like teleconnections over the Indo‐Pacific basin.