Radiocarbon Variability Research Articles

Radiocarbon as a Dating Tool and Tracer in Paleoceanography

AbstractRadiocarbon is an extremely useful carbon cycle tracer and radiometric dating tool. Here, we review the main principles and challenges involved in the use of radiocarbon in paleoceanography. First, we present a conceptual framework in which there are three possible uses of a radiocarbon measurement: (a) to obtain a calendar age interval, or a fossil entity's age; (b) to obtain an estimate of a carbon reservoir's past radiocarbon activity; or (c) to compare the relative radiocarbon activities of two contemporary carbon reservoirs. We discuss the analysis of marine fossil material, the generation of an atmospheric reference curve, and the interpretation of marine radiocarbon “ventilation metrics” in relation to this reference curve. It is emphasized that marine radiocarbon integrates the influences of: changing radiocarbon production; air‐sea gas exchange effects at the sea surface; transport times within the ocean interior; and the mixing of water parcels with different transit times from the sea surface, and different sea‐surface sources. These controls are what make radiocarbon such a powerful paleoceanographic tracer, though the difficulty of disentangling them is what makes marine radiocarbon dating and tracer studies so challenging. We discuss the implementation of radiocarbon in numerical models, and explore the theory linking ocean‐atmosphere partitioning of radiocarbon and CO2. Finally, we review existing records of marine radiocarbon variability over the last ∼25,000 years, which highlight the influence of ocean‐atmosphere carbon exchange on past atmospheric CO2 and climate, and point to emerging opportunities for resolving the global radiocarbon‐ and carbon budgets over the last glacial cycle.

A 250‐Year, Decadally Resolved, Radiocarbon Time History in the Gulf of Maine Reveals a Hydrographic Regime Shift at the End of the Little Ice Age

AbstractIn order to document relative changes in water mass contributions in the Gulf of Maine (GoM), we used the shell material of the long‐lived ocean quahog (Arctica islandica). A multicentury, crossdated master shell growth chronology facilitated the reconstruction of a radiocarbon Δ14C history prior to the radiocarbon bomb‐pulse of the 1950s. This reconstruction reveals a highly variable Δ14C series (mean = −56.6 ± 8.0‰ (1σ); N = 34) from CE 1685 to 1935. Δ14C values indicate a rapid shift ca. 1860 CE in source waters to the GoM. From CE 1685 to 1860, GoM waters were dominated by an admixture of Warm Slope Water primarily composed of tropical Atlantic surface waters/Gulf Stream Waters, and Scotian Shelf Water. This water regime was followed by a rapid Δ14C transition to a Labrador Slope Water endmember after CE 1860, with an apparent decrease in Scotian Shelf Water. Together, this shift is likely related to broader changes in the Arctic and the Labrador Sea, and a short‐term strengthening of the Atlantic meridional overturning circulation. Labrador Slope Water dominating GoM hydrography in the 1900s is verified by the similarities between this record and other coral‐ and shell‐derived Δ14C records influenced by waters with Labrador Sea origin. This suggests that GoM radiocarbon variability broadly reflects large‐scale ocean circulation processes in the Northwestern Atlantic. The lack of Δ14C values much below the Labrador Slope Water endmember suggests that the interior GoM gets very little to no Antarctic Intermediate Water as other studies had previously suggested.

Climate-induced speleothem radiocarbon variability on Socotra Island from the Last Glacial Maximum to the Younger Dryas

Abstract. In this study, the dead carbon fraction (DCF) variations in stalagmite M1-5 from Socotra Island in the western Arabian Sea were investigated through a new set of high-precision U-series and radiocarbon (14C) dates. The data reveal an extreme case of very high and also climate-dependent DCF. For M1-5, an average DCF of 56.2±3.4 % is observed between 27 and 18 kyr BP. Such high DCF values indicate a high influence of aged soil organic matter (SOM) and nearly completely closed-system carbonate dissolution conditions. Towards the end of the last glacial period, decreasing Mg∕Ca ratios suggest an increase in precipitation which caused a marked change in the soil carbon cycling as indicated by sharply decreasing DCF. This is in contrast to the relation of soil infiltration and DCF as seen in stalagmites from temperate zones. For Socotra Island, which is influenced by the East African–Indian monsoon, we propose that more humid conditions and enhanced net infiltration after the Last Glacial Maximum (LGM) led to dense vegetation and thus lowered the DCF by increasing 14CO2 input into the soil zone. At the onset of the Younger Dryas (YD) a sudden change in DCF towards much higher, and extremely variable, values is observed. Our study highlights the dramatic variability of soil carbon cycling processes and vegetation feedback on Socotra Island manifested in stalagmite DCF on both long-term trends and sub-centennial timescales, thus providing evidence for climate influence on stalagmite radiocarbon. This is of particular relevance for speleothem studies that aim to reconstruct past atmospheric 14C (e.g., for the purposes of 14C calibration), as these would rely on largely climate-independent soil carbon cycling above the cave.

New Evidence for a Mid- to Late-Holocene Change in the Marine Reservoir Effect Across the South Pacific Gyre

AbstractHolocene climate change in the South Pacific is of major interest to archaeologists and Quaternary researchers. Regional surface ocean radiocarbon (14C) values are an established proxy for studying changing oceanographic and climatic conditions. Unfortunately, radiocarbon variability in the marine environment over the period of specific importance to human colonization of the remote Pacific islands—the last 3500 years—has been poorly studied. In order to build robust and accurate archaeological chronologies using shell, it is important to rectify this. In this paper, radiocarbon marine reservoir offsets (ΔR) are presented from eight archaeological sites, ranging in age from 350 cal BP to 3000 cal BP, and compared to coral datasets from the east Australian coastline. The results indicate that a significant decrease in the South Pacific Gyre ΔR occurred between 2600 and 2250 cal BP, most likely caused by changes in ocean circulation and climate. Accurately recording the timing of variability in reservoir offset is critical to untangling changes in society that took place in the Pacific, in particular, the development of Ancestral Polynesian Society.

Dimensions of Radiocarbon Variability within Sedimentary Organic Matter

ABSTRACTOrganic carbon (OC) radiocarbon (14C) signatures in marine surface sediments are highly variable and the causes of this heterogeneity remain ambiguous. Here, we present results from a detailed 14C-based investigation of an Arabian Sea sediment, including measurements on organic matter (OM) in bulk sediment, specific grain size fractions, and OC decomposition products from ramped-pyrolysis-oxidation (RPO). Our results show that 14C ages of OM increase with increasing grain size, suggesting that grain size is an important factor controlling the 14C heterogeneity in marine sediments. Analysis of RPO decomposition products from different grain size fractions reveals an overall increase in age of corresponding thermal fractions from finer to coarser fractions. We suggest that hydrodynamic properties of sediment grains exert the important control on the 14C age distribution of OM among grain size fractions. We propose a conceptual model to account for this dimensionality in 14C variability that invokes two predominant modes of OM preservation within different grain size fractions of Arabian Sea sediment: finer (<63 µm) fractions are influenced by OM-mineral grain aggregation processes, giving rise to relatively uniform 14C ages, whereas OM preserved in coarser (>63 µm) fractions includes materials encapsulated within microfossils and/or entrained fossil (14C-depleted) OC hosted in detrital mineral grains. Our findings highlight the value of RPO for assessment of 14C age variability in sedimentary OC, and for assessing mechanisms of OM preservation in aquatic sediments.

Millennial-scale variability in the local radiocarbon reservoir age of south Florida during the Holocene

Abstract A growing body of research suggests that the marine environments of south Florida provide a critical link between the tropical and high-latitude Atlantic. Changes in the characteristics of water masses off south Florida may therefore have important implications for our understanding of climatic and oceanographic variability over a broad spatial scale; however, the sources of variability within this oceanic corridor remain poorly understood. Measurements of ΔR, the local offset of the radiocarbon reservoir age, from shallow-water marine environments can serve as a powerful tracer of water-mass sources that can be used to reconstruct variability in local-to regional-scale oceanography and hydrology. We combined radiocarbon and U-series measurements of Holocene-aged corals from the shallow-water environments of the Florida Keys reef tract (FKRT) with robust statistical modeling to quantify the millennial-scale variability in ΔR at locations with (“nearshore”) and without (“open ocean”) substantial terrestrial influence. Our reconstructions demonstrate that there was significant spatial and temporal variability in ΔR on the FKRT during the Holocene. Whereas ΔR was similar throughout the region after ∼4000 years ago, nearshore ΔR was significantly higher than in the open ocean during the middle Holocene. We suggest that the elevated nearshore ΔR from ∼8000 to 5000 years ago was most likely the result of greater groundwater influence associated with lower sea level at this time. In the open ocean, which would have been isolated from the influence of groundwater, ΔR was lowest ∼7000 years ago, and was highest ∼3000 years ago. We evaluated our open-ocean model of ΔR variability against records of local-to regional-scale oceanography and conclude that local upwelling was not a significant driver of open-ocean radiocarbon variability in this region. Instead, the millennial-scale trends in open-ocean ΔR were more likely a result of broader-scale changes in western Atlantic circulation associated with an increase in the supply of equatorial South Atlantic water to the Caribbean and shifts in the character of South Atlantic waters resulting from variation in the intensity of upwelling off the southwest coast of Africa. Because accurate estimates of ΔR are critical to precise calibrations of radiocarbon dates from marine samples, we also developed models of nearshore and open-ocean ΔR versus conventional 14 C ages that can be used for regional radiocarbon calibrations for the Holocene. Our study provides new insights into the patterns and drivers of oceanographic and hydrologic variability in the Straits of Florida and highlights the value of the paleoceanographic records from south Florida to our understanding of Holocene changes in climate and ocean circulation throughout the Atlantic.

Spatiotemporal Variability in ΔR in the Northern Gulf of Mexico, USA

AbstractStrombus alatus and Busycon sinistrum are large marine gastropods that are frequently recovered from archaeological contexts in southeastern North America. We previously proposed a reservoir age offset (ΔR) for B. sinistrum from the northern Gulf of Mexico region based on known-age pre-bomb 20th-century specimens. We also reported significant variability in radiocarbon both among and within S. alatus specimens, which precluded a reliable estimation of ΔR for this taxon. In this paper, we present a complementary data set from archaeological contexts to re-evaluate marine reservoir effects in the northern Gulf Coast region at multiple spatial and temporal scales. The new data set consists of a total of 13 14C age determinations from well-associated marine (B. sinistrum and S. alatus) and terrestrial (Odocoileus virginianus) samples from a closed context at the Bayou St. John (1BA21) archaeological site. We suggest a slightly updated ∆R value of –2±53 14C yr for late Holocene-age B. sinistrum from the northern Gulf Coast region. S. alatus, and possibly other species of strombid conchs, are poor candidates for 14C dating due to the highly variable 14C content observed within and among specimens. Though subregional variability in inputs of 14C-depleted waters is likely, life-history factors related to ontogenetic niche and/or habitat shifts appear to be a major influence in shell 14C for S. alatus.

<sup>14</sup>C in cropland soil of a long-term field trial – experimental variability and implications for estimating carbon turnover

Abstract. Because of their controlled nature, the presence of independent replicates, and their known management history, long-term field experiments are key to the understanding of factors controlling soil carbon. Together with isotope measurements, they provide profound insight into soil carbon dynamics. For soil radiocarbon, an important tracer for understanding these dynamics, experimental variability across replicates is usually not accounted for; hence, a relevant source of uncertainty for quantifying turnover rates is missing. Here, for the first time, radiocarbon measurements of five independent field replicates, and for different layers, of soil from the 66-year-old controlled field experiment ZOFE in Zurich, Switzerland, are used to address this issue. 14C variability was the same across three different treatments and for three different soil layers between the surface and 90 cm depths. On average, experimental variability in 14C content was 12 times the analytical error but still, on a relative basis, smaller than variability in soil carbon concentration. Despite a relative homogeneous variability across the field and along the soil profile, the curved nature of the relationship between radiocarbon content and modelled carbon mean residence time implies that the absolute error of calculated soil carbon turnover time increases with soil depth. In our field experiment findings on topsoil carbon turnover variability would, if applied to subsoil, tend to underweight turnover variability even if experimental variability in the subsoil isotope concentration is the same. Together, experimental variability in radiocarbon is an important component in an overall uncertainty assessment of soil carbon turnover.

Variability in Radiocarbon Dates in Middle Pleniglacial Wood from Kurtak (Central Siberia)

The chronology of long Upper Pleistocene loess sequences in Eurasia is based on combined pedostratigraphy and radiocarbon dating of high-quality charcoal. The accuracy of such a chronology depends on the reproducibility and precision of the 14C dates. However, certain dates may show discrepancies with regard to their chronostratigraphic context based on series of coherent dates. In order to evaluate the consistency and variation in the 14C dates obtained from small charcoal pieces, this question was tested on a set of spruce wood remains with well-preserved tree rings found in the Middle Pleniglacial loess-loam sequence of Kurtak (central Siberia). Tree-ring analysis of five fairly large wood pieces from three successive layers, dated to about 30.0, 30.8, and 32.2–32.5 ka BP previously, was done by continuous sampling of 90–150 rings on each wood piece. This enabled direct comparison of the succession of tree rings with the 14C dates. A total of 133 dates was obtained for the five wood pieces. The results show fluctuations in the 14C dates within a time range between 1000 and 2000 yr. Four possible causes for such variation will be discussed herein: (1) internal variability of the AMS dating method; (2) outliers; (3) variations in the 14C background; and (4) external factors such as past atmospheric 14C variations.

Foraminiferal radiocarbon record of northeast Pacific decadal subsurface variability

The decadal dynamics of the subsurface North Pacific Ocean are largely inaccessible beyond sparse instrumental observations spanning the last 20 years. Here we present a ∼200 year long record of benthic foraminiferal radiocarbon (Δ14C), extracted at biennial resolution from the annually laminated sediments at the Santa Barbara Basin (SBB) depocenter (∼600 m). The close match between core top benthic foraminiferal Δ14C values and the Δ14C of seawater dissolved inorganic carbon (DIC) suggests that benthic foraminifera faithfully capture the bottom water radiocarbon concentrations, as opposed to that of the deeper (>0.5 cm) sediment porewater zone. The full time series of benthic foraminiferal Δ14C displays significant variability on decadal timescales, with excursions on the order of 40‰. These excursions are overprinted by a unidirectional trend over the late 20th century that likely reflects the sedimentary incorporation of bomb radiocarbon (via remineralized particulate organic carbon). We isolate this trend by means of a one‐dimensional oxidation model, which considers the possible contribution of remineralized particles to the total ambient carbon pool. This oxidation model also considers the possible influence of carbon with a variety of sources (ages). Though variable oxidation of preaged carbon could exert a strong influence on benthic foraminiferal radiocarbon variability, the totality of evidence points to the vertical density structure along the Southern California Margin (SCM) as the primary driver of the SBB benthic foraminiferal Δ14C record. For example, intervals characterized by significantly lower Δ14C values correspond to periods of enhanced upwelling and subsurface equatorward flow along the SCM.

Radiocarbon Variability in Groundwater in an Extremely Arid Zone—The Arava Valley, Israel

Radiocarbon values of groundwater in main aquifers of the extremely arid Negev Desert and the Arava Valley, southern Israel, are used for studying the underground flow regime, particularly the complex connections between different aquifers and mixing of water bodies. The study shows that 14C can serve as a hydrological tracer in arid environments and that groundwater dating may be possible (although not very accurate) even in this extremely arid environment (precipitation, <50 mm/yr), where there is almost no vegetation. There are several aquifers in this region, some of which are deep (deeper than 500 m) and regional and contain mainly fossil water, while others are local and restricted to the Arava, much shallower (50–200 m) and are thought to contain historical to recent waters. Most of the current recharge to these shallow unconsolidated aquifers comes from flash floods that flow from the mountains rising on both sides of the valley. The groundwater in the deep aquifers has low 14C values (usually <5 pMC), implying old ages (preliminary ages >26,000 yr). Groundwater in the shallow aquifers characterized by higher 14C values (up to 60–70 pMC) imply younger ages and faster groundwater flow (recent recharge). This is also supported by the presence of tritium in some of the samples. A few exceptional values are explained by the unique mixing of water from different sources; another is due to a technical failure in the well.

Radiocarbon Variability in Groundwater in an Extremely Arid Zone—The Arava Valley, Israel

Radiocarbon values of groundwater in main aquifers of the extremely arid Negev Desert and the Arava Valley, southern Israel, are used for studying the underground flow regime, particularly the complex connections between different aquifers and mixing of water bodies. The study shows that 14C can serve as a hydrological tracer in arid environments and that groundwater dating may be possible (although not very accurate) even in this extremely arid environment (precipitation, <50 mm/yr), where there is almost no vegetation. There are several aquifers in this region, some of which are deep (deeper than 500 m) and regional and contain mainly fossil water, while others are local and restricted to the Arava, much shallower (50–200 m) and are thought to contain historical to recent waters. Most of the current recharge to these shallow unconsolidated aquifers comes from flash floods that flow from the mountains rising on both sides of the valley. The groundwater in the deep aquifers has low 14C values (usually <5 pMC), implying old ages (preliminary ages >26,000 yr). Groundwater in the shallow aquifers characterized by higher 14C values (up to 60–70 pMC) imply younger ages and faster groundwater flow (recent recharge). This is also supported by the presence of tritium in some of the samples. A few exceptional values are explained by the unique mixing of water from different sources; another is due to a technical failure in the well.

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

Late Holocene radiocarbon variability in Northwest Atlantic slope waters

Deep-sea gorgonian corals secrete a 2-part skeleton of calcite, derived from dissolved inorganic carbon at depth, and gorgonin, derived from recently fixed and exported particulate organic matter. Radiocarbon contents of the calcite and gorgonin provide direct measures of seawater radiocarbon at depth and in the overlying surface waters, respectively. Using specimens collected from Northwest Atlantic slope waters, we generated radiocarbon records for surface and upper intermediate water layers spanning the pre- and post-bomb- 14C eras. In Labrador Slope Water (LSW), convective mixing homogenizes the pre-bomb Δ 14C signature (− 67 ± 4‰) to at least 1000 m depth. Surface water bomb- 14C signals were lagged and damped (peaking at ∼ + 45‰ in the early 1980s) relative to other regions of the Northwest Atlantic, and intermediate water signals were damped further. Off southwest Nova Scotia, the vertical gradient in Δ 14C is much stronger. In surface water, pre-bomb Δ 14C averaged − 75 ± 5‰. At 250–475 m depth, pre-bomb Δ 14C oscillated quasi-decadally between − 80 and − 100‰, likely reflecting interannual variability in the presence of Labrador Slope Water vs. Warm Slope Water (WSW). Finally, subfossil corals reveal no systematic changes in vertical Δ 14C gradients over the last 1200 yr.

A review of some attacks on the overkill hypothesis, with special attention to misrepresentations and doubletalk

This paper addresses misrepresentations and errors in attacks directed against the Overkill hypothesis that was proposed by Paul Martin to explain selective late Pleistocene extinctions. The opposing Climate-Change hypothesis to explain extinctions is driven by ideology as much as by objective reasoning because it is repeated so frequently without strong new evidence to support it, but it has failed to nail down a victory in public opinion. Overkill, which is not an anti-climate-change hypothesis, is perhaps too “flexible” to persuade all scientists, especially because negative evidence (a lack of megafaunal killsites) is considered to be as corroborative as positive evidence. Multiphase models of extinctions that propose different genera died out at different times have become less and less likely as more radiometric dating is done, and overlook the documented variability in atmospheric radiocarbon around the end of the Pleistocene, which prevents the high resolution chronology-building necessary to support a discontinuous model of the extinctions. The extinctions were geologically abrupt, selective, and unique, and therefore they require unique explanations.

Radiocarbon Variability in the Western North Atlantic During the Last Deglaciation

We present a detailed history of glacial to Holocene radiocarbon in the deep western North Atlantic from deep-sea corals and paired benthic-planktonic foraminifera. The deglaciation is marked by switches between radiocarbon-enriched and -depleted waters, leading to large radiocarbon gradients in the water column. These changes played an important role in modulating atmospheric radiocarbon. The deep-ocean record supports the notion of a bipolar seesaw with increased Northern-source deep-water formation linked to Northern Hemisphere warming and the reverse. In contrast, the more frequent radiocarbon variations in the intermediate/deep ocean are associated with roughly synchronous changes at the poles.

Pre-Bomb Radiocarbon Variability Inferred from a Kenyan Coral Record

We report results from AMS radiocarbon measurements (δ14C) in corals recovered off the coast of Kenya. Bimonthly samples which span the pre-bomb era average −51 (±3.7; n=43), when age and Suess effect are corrected, and over the time of interest (1946–1954) do not exhibit any discernible seasonality. Relative to regional pre-bomb δ14C values in the western Indian Ocean, our results indicate 14C enrichment off the coast of Kenya. Furthermore, the absence of a distinct subannual δ14C signal suggests that open and coastal upwelling is negligible off the coast of Kenya. Unlike pre-bomb values south of the equator near Seychelles and Madagascar, our pre-bomb value are enriched by more than 10. The enrichment of pre-bomb Kenyan δ14C values relative to sites around Mauritius, northern Madagascar and Seychelles, suggest that the influence of depleted δ14C water transported in the SEC is limited to regions south of 3 to 4°S.

Radiocarbon variability in the western equatorial Pacific inferred from a high‐resolution coral record from Nauru Island

We have generated a high resolution coral Δ14C record spanning the last 50 years to document the seasonal and interannual redistribution of surface waters in the western tropical Pacific. Prebomb (1947–1956) Δ14C values average −63‰ and have a total range of 30‰. Values begin to increase in 1957, reaching a maximum of 137‰ in mid‐1983. Large interannual variability of up to 80‰ closely follows the El Niño‐Southern Oscillation (ENSO). During each ENSO warm phase, Δ14C values begin to increase, reflecting the reduction of low‐14C water upwelling in the east and the invasion of subtropical water into the western equatorial tropical Pacific. Maximum Δ14C values are in phase or lag the corresponding sea surface temperature maxima in the eastern tropical Pacific, whereas the rapid return to more negative Δ14C is in phase with eastern Pacific ENSO indices. The highest‐amplitude excursions occur during the 1965/1966 and 1972/1973 events, when the 14C contrast is highest between the eastern Pacific and subtropics. The 1982/1983 El Niño, although a larger ENSO event, has a lower Δ14C amplitude, reflecting the penetration of bomb radiocarbon into the equatorial undercurrent and the reduced contrast in Δ14C between thermocline and subtropical surface waters at that time. This coral record demonstrates the potential for using similar radiocarbon time series for documenting variability in Pacific shallow circulation over interannual and decadal timescales.

Distributions of dissolved organic and inorganic carbon and radiocarbon in the eastern North Pacific continental margin

Temporal variations in the natural radiocarbon ( 14C) signatures of dissolved organic and inorganic carbon (DOC and DIC, respectively) in seawater have been studied previously (Druffel, E.R.M., Bauer, J.E., Williams, P.M., Griffin, S., Wolgast, D.M., 1996. Seasonal variability of radiocarbon in particulate organic carbon in the northeast Pacific. J. Geophys. Res. 101, 20 543–20 552; Bauer, J.E., Druffel, E.R.M., Williams, P.M., Wolgast, D.M., Griffin, S., 1998. Temporal variability in dissolved organic carbon and radiocarbon in the eastern North Pacific Ocean. J. Geophys. Res. 103, 2867–2882) at a long-term time-series station (Sta. M: 32°N, 123W) in the eastern North Pacific located at the eastern edge of the North Pacific abyssal plain. In June 1995 a transect was made from Sta. M inshore to approximately 500 m depth in order to evaluate the distributions of 14C in DOC and DIC from the abyssal plain to the upper continental slope. Concentrations and Δ 14C values of DOC in mixed layer waters (25 and 85 m) decreased toward the upper slope. In deeper waters, concentrations and Δ 14C values were in general similar at all three sites. Differences in DOC concentrations and Δ 14C-DOC between Sta. M and the rise and upper slope sites were explained in part by the mixing of DOC and Δ 14C along constant density ( σ t) surfaces. However, specific deviations from conservative behavior due to mixing were observed for Δ 14C-DOC at mesopelagic (∼700 m) and near-bottom (∼3600– 3900 m) depths of the continental rise. Comparable findings are reported for DIC, where σ t-normalized concentrations and Δ 14C values in Sta. M, rise and upper slope waters were similar, with the exception of slight increases in concentrations and Δ 14C values in near-bottom waters of the rise. These observations indicate that both DOC and DIC in continental rise and slope surface waters of the eastern North Pacific Ocean margin are comprised of a component of actively upwelled material derived from deeper offshore waters and a component of recent surface-derived material. Our data suggest that contributions of both 14C-enriched and 14C-depleted DOC and DIC to localized regions of the water column occur in waters of the continental rise and slope. The sources of material having anomalous Δ 14C values are not certain, but may include organic matter derived from off-shelf and slope transport, particulate organic carbon solubilization, and sediment porewaters. We also examine the relationships between Δ 14C-DOC and both Δ 14C-DIC and Δ 14C of suspended POC measured in a companion study as a means of evaluating similarities or differences in the mechanisms responsible for the distributions of each at all three sites.

Physical controls on dissolved inorganic radiocarbon variability in the California Current

We present depth profiles of Δ 14C and δ 13C of dissolved inorganic carbon (DIC) at Station M in the Eastern North Pacific. Several seasonal profiles are presented for the time period between 1991 and 1996. Comparison with GEOSECS data clearly shows changes in ocean radiocarbon profiles since 1973. The Δ 14C of DIC shows the most variability at depths of 450, 85, and 25 m, and the lowest variability at depths of 1600 and 2500 m. The largest variability in DIC Δ 14C occurs at 450 m, a depth marked by large fluctuations in the radiocarbon signatures of the source waters. The likely controls of DIC Δ 14C variability are physical changes in the circulation of the California Current System. A simple two-box model is used to show the importance of wind driven mixing at the surface. We discuss the likely effects of mesoscale eddies and ENSO on the DIC Δ 14C values at this site. We also show that remineralization of organic carbon (dissolved or particulate) is not responsible for the variability in the Δ 14C of DIC observed at Station M.