. Radiocarbon (∆14C) measurements of monthly samples from a Galapagos surface coral are among the first
data sets from the new Keck Carbon Cycle Accelerator Mass Spectrometry laboratory at the University of California, Irvine.
An average ∆14C value of –62‰ is obtained for 144 measurements of samples from monthly coral bands that lived from
about AD 1760–1771 (±6 yr). High ∆14C values were found during January through March, when upwelling was weak or
absent at the Galapagos Islands. Low ∆14C values were obtained mid-year during strong upwelling. The average seasonal
variability of ∆14C was 15–25‰, which is greater than that at other tropical and subtropical locations in the Pacific Ocean
because of intense seasonal upwelling at this site. Periods of sustained high ∆14C values were found during 1762–1763 and
1766. A spectral analysis revealed that the spectral density for the ∆14C data displays most of its variance at the 5-yr cycle,
which is reflective of El Niño periodicity during the 20th century

Beaupre, SR

Druffel, ER

Druffel-Rodriguez, KC

Griffin, S

Hwang, J

Komada, T

Santos, GM

Southon, J

eScholarship - University of California

UC IrvineUC Irvine Previously Published WorksTitleVariability of radiocarbon during the 1760s in monthly corals from the Galapagos IslandsPermalinkhttps://escholarship.org/uc/item/3jk27557JournalRadiocarbon, 46(2)AuthorsDruffel, ERDruffel, ERGriffin, Set al.Publication Date2004Copyright InformationThis work is made available under the terms of a Creative Commons Attribution License, availalbe at https://creativecommons.org/licenses/by/4.0/ Peer reviewedeScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaRADIOCARBON, Vol 46, Nr 2, 2004, p 627–631  © 2004 by the Arizona Board of Regents on behalf of the University of Arizona© 2004 by the Arizona Board of Regents on behalf of the University of ArizonaProceedings of the 18th International Radiocarbon Conference, edited by N Beavan Athfield and R J SparksRADIOCARBON, Vol 46, Nr 2, 2004, p 627–631627VARIABILITY OF MONTHLY RADIOCARBON DURING THE 1760s IN CORALS FROM THE GALAPAGOS ISLANDSEllen R M Druffel1 • Sheila Griffin • Jeomshik Hwang • Tomoko Komada • Steven R Beaupre • Kevin C Druffel-Rodriguez • Guaciara M Santos • John SouthonDepartment of Earth System Science, University of California, Irvine, California 92697, USA.ABSTRACT. Radiocarbon (∆14C) measurements of monthly samples from a Galapagos surface coral are among the firstdata sets from the new Keck Carbon Cycle Accelerator Mass Spectrometry laboratory at the University of California, Irvine.An average ∆14C value of –62‰ is obtained for 144 measurements of samples from monthly coral bands that lived fromabout AD 1760–1771 (±6 yr). High ∆14C values were found during January through March, when upwelling was weak orabsent at the Galapagos Islands. Low ∆14C values were obtained mid-year during strong upwelling. The average seasonalvariability of ∆14C was 15–25‰, which is greater than that at other tropical and subtropical locations in the Pacific Oceanbecause of intense seasonal upwelling at this site. Periods of sustained high ∆14C values were found during 1762–1763 and1766. A spectral analysis revealed that the spectral density for the ∆14C data displays most of its variance at the 5-yr cycle,which is reflective of El Niño periodicity during the 20th century. INTRODUCTIONData from banded corals show that the radiocarbon (14C) “age” of pre-bomb, non-polar surfaceocean ranges from 620 14C yr (–72‰) in the eastern tropical Pacific to 340 14C yr (–40‰) in themid-gyre regions of the Pacific Ocean (Druffel 1987; Druffel et al. 2001). These non-contemporaryreservoir ages of surface dissolved inorganic carbon (DIC) are due mainly to vertical and advectivemixing of subsurface water having even “older” 14C ages because of radioactive decay during isola-tion from the atmosphere. Because 14C has a long turnover time (10 yr) in the atmosphere withrespect to the transfer to the sea surface, ∆14C of DIC in surface seawater is a tracer of circulationpatterns and thermocline depth. Records of upper ocean circulation changes during the past areessential for understanding present and future climate changes. Corals reflect the ∆14C in surface DIC and provide time histories of ∆14C on timescales of monthsto millennia. Records of ∆14C in the surface ocean reveal past variability of surface-subsurfacemixing and horizontal current shifts. We report monthly ∆14C measurements from a Galapagos coralrepresenting the 11-yr period AD 1760–1771 (±6 yr). We show that ∆14C is high during thebeginning of the year, when trade winds and upwelling were weak, and low ∆14C values during therest of the year, when wind-driven upwelling was strong.OCEANOGRAPHIC SETTING AND SAMPLE COLLECTIONThe coral sequence was collected by Drs Rob Dunbar, Mitch Colgan, and Gerard Wellington froma dead, massive head of Pavona clavus in Urvina Bay on the west coast of Isabela Island (0°15′S,91°22′W) in the Galapagos archipelago in 1986 (see Dunbar et al. 1991 for details). The coral sitewas directly in the path of the cold, nutrient-rich Peru Current and the Equatorial Counter Current(Cromwell Current), which brings subsurface water eastward. The coral and the shoreline of thecentral coast of Urvina Bay was raised above sea level in March 1954 because of the intrusion andupward movement of magma below the surface (Richards 1957). 1Corresponding author. Email: edruffel@uci.edu.628 E R M Druffel et al.METHODSDunbar et al. (1991) cleaned, X-rayed, and developed an age model for this coral sequence usinghigh-density bands that accrete from about January to April of each year. Based on preliminary230Th mass spectrometric analyses performed by Bruno Hamelin (LDEO), Dunbar et al. (1991) esti-mated an uncertainty of ±10 yr for ages at the base of the section (AD 1583), which translates to±6 yr for our samples during the 1760s (assuming the uncertainty is linear).  We performed ∆14C measurements on coral samples that had been ground off with a diamond-tipdremel tool to a width of 1 mm each from a vertical strip of coral (10 mm wide and 7 mm thick). Thesequence was 144 mm long and spanned the time period from late 1760 to mid-1771; the averagesampling frequency was 13 samples/yr.Sub-samples (8 mg each) were converted to CO2 in evacuated Vacutainers® with 85% phosphoricacid, then to graphite on Co catalyst with H2 gas as the reducing agent (Vogel et al. 1987). All of thesamples were analyzed for 14C/12C using accelerator mass spectrometry (AMS) at the Keck CarbonCycle AMS Laboratory at UC Irvine. Background subtraction was applied using 14C-free spar cal-cite. The ∆14C measurements had a total uncertainty (counting statistics and laboratory reproducibil-ity) of ±4–5‰ (1 σ). Radiocarbon measurements are reported as ∆14C values (for geochemical sam-ples with known age) according to Stuiver and Polach (1977). The ∆14C results were corrected usingδ13C values measured either by the AMS or on CO2 splits prior to graphitization.RESULTSThe average of 144 monthly ∆14C measurements (duplicate analyses and splits of the same CO2were averaged to a single point) from the Urvina Bay coral is –62 ± 6 (sd)‰. The range of the indi-vidual values is large (33‰), from –80‰ in early 1765 to –47‰ in mid-1770 (Figure 1). A least-squares fit of all monthly ∆14C measurements reveals no significant trend with time. Seasonal cycles are apparent in the ∆14C data during most years (Figure 1), though these cycles arenot well defined. The highest ∆14C values (large grey circles) are at the base of the high-densitybands (vertical lines in Figure 1), which are formed when surface waters are warm and upwelling isweak or absent (January through March) (Druffel 1981). The average of these 10 high ∆14C valuesis –53 ± 3‰. Most of the lowest ∆14C values (large black circles) were from samples in the middleof the bands. The average of the 11 low ∆14C values is –71 ± 5‰. A least-squares fit of the low ∆14Cvalues reveals a 6‰ decrease over the 11-yr time period, that for the high ∆14C values is notsignificant.DISCUSSIONThere appears to be an interannual fluctuation of the ∆14C values in this record. The ∆14C values for1765 and 1769 were lower than those for other years. Six to 7 consecutive high ∆14C values werefound for 1766 and 1770 (average –54‰ for both). A smoothed line of the monthly data is shown inFigure 1 using a Stineman function and suggests that, based on ∆14C data alone, El Niño eventsoccurred during 1762–1763, 1766, and possibly 1770.  During 1762–63, ∆14C values increased dur-ing the upwelling season relative to other years, similar to Guilderson and Schrag’s (1998) ∆14Cincrease in Galapagos coral after the climate shift of 1976.Periodicities within the monthly ∆14C record were investigated by spectral analysis using Spectralsoftware provided by Phil Howell (personal communication, 2000). The spectral density for the∆14C data displays a small amount of variance at the 1-yr cycle and most of its variance at the 5-yrVariability of 14C in Corals from the Galapagos Islands 629cycle (significant to the 95% confidence level). The 5-yr cycle, however, is close to the nyquist fre-quency of 5.5 yr (11 yr total record/2), so the significance of this result is questionable. Nonetheless,a 5-yr cycle is similar to the frequency of El Niño events during the 20th century, which would sug-gest that, within the limits of this short data set, El Niño (high ∆14C) events during the 1760s werenot different in frequency from those that were observed during recent times.  The average seasonal amplitude of ∆14C values for the Galapagos coral during the 1760s was 15 to25‰ (Figure 1), whereas those reported for recent corals from other Pacific sites are significantlylower (3–4‰ for Hawaiian coral, Druffel et al. 2001; 10–15‰ for Raratonga coral, Guilderson et al.2000; 10–15‰ for Fanning Island coral, Grottoli et al. 2003). Seasonal upwelling at the easterntropical Pacific is strong during the months of July through September, when the southeast tradewinds are strongest. During this period, cold (18–20 °C), nutrient-rich water upwells into the PeruCurrent and Equatorial Counter Current, both of which lave the Galapagos Islands. The lowest pre-bomb ∆14C values of any non-polar surface location are found at the Galapagos (Druffel 1981;Toggweiler et al. 1991) because of its proximity to intense equatorial and coastal upwelling.A comparison of average pre-bomb ∆14C values for corals from several Pacific locations revealsthat the monthly Galapagos record for the 1760s (–62 ± 6‰) and the annual Galapagos record for1930–1941 (Druffel 1989; –70 ± 6‰) were the lowest. Monthly samples from a Fanning Islandcoral (4°N, 159°W) have an average ∆14C value of –55 ± 5‰ (n = 217) between 1922 and 1947(Grottoli et al. 2003). In the western Pacific, Guilderson et al. (1998) determined the average ofmonthly ∆14C values at Nauru (1°S, 166°E) was –58‰ between 1947 and 1956. An average of allHawaiian bimonthly ∆14C analyses for the time period 1920.8–1923.0 was –46.4 ± 2.3‰ (DruffelFigure 1 ∆14C measurements of monthly coral bands from Urvina Bay, Galapagos, during the period 1760 to 1771. Theuncertainty of the age assignments in this age range is ±6 yr. Duplicate analyses of separate aliquots from the same coralsample and splits of the same CO2 were averaged to a single point. Large grey points represent the highest ∆14C valuesfor each year (lowest upwelling) and large black points represent the lowest ∆14C values for each year (most intenseupwelling). Vertical lines mark the base of the high-density bands, which are generally formed when upwelling is low.Horizontal lines (grey) are the least-squares fits of the high and low 14C values, and that (black) for all of the monthlyresults. The smoothed line of the monthly data is obtained using a Stineman function, which suggests that El Niño eventsoccurred during 1762–63 and possibly 1766. -90-80-70-60-50-400 20 40 60 80 100 120 140 ²14Cmm growth1762 1764 1770176817662 sigmaerrorEl Niño El Niño El Niño∆14 C630 E R M Druffel et al.et al. 2001). A coral from the subtropical South Pacific (Rarotonga, 21°S, 160°W) had a pre-bomb,average ∆14C value of –52‰ for the 1950s (Guilderson et al. 2000). These sites have higher ∆14Clevels because their mixed layers are more stable and there is little or no upwelling compared to theintense upwelling conditions in the eastern equatorial Pacific (Toggweiler et al. 1991).The physical mechanism that causes temporal variability of climate and ∆14C on an interannualtimescale is El Niño/Southern Oscillation (ENSO). ENSO is caused by interaction of the tropicalocean and atmosphere (Bjerknes 1969). 14C has been shown to increase in Galapagos coral bands thatgrew during El Niño events of the past 40 yr because upwelled waters no longer reached the sea sur-face (Druffel 1981; Brown et al. 1993), especially during mid-year (Guilderson and Schrag 1998).These events were generally coincident with warm waters that laved the region. Therefore, in thecase of El Niño events, high ∆14C measurements reflect a cessation of local upwelling, the presenceof high 14C surface water advected eastward along the Equator from the western warm pool, andtransport of off-equatorial water masses due to the southward migration of the ITCZ during El Niñoevents. We suggest that these conditions prevailed during 1762–63, 1766, and part of 1770. CONCLUSIONSRadiocarbon (∆14C) measurements of monthly samples from a Galapagos surface coral show anaverage ∆14C value of –62‰ for samples that lived during ~1760–1771. Periods of sustained high∆14C values were found for the years 1762–63 and possibly 1770. Spectral analysis of the ∆14C datashows that the spectral density has most of its variance in the 5-yr cycle, reflecting the El Niño cycle;some variance is seen for the annual cycle. The 5-yr cycle is similar to the frequency of El Niñoevents during the 20th century, which would indicate that El Niño (high ∆14C) events during the1760s occurred at a frequency similar to that during the last 100 yr. The seasonal variability of ∆14Cduring the 1760s, though not well defined, had a higher amplitude range than at other sites in thePacific because of the strong seasonal upwelling that is unique to the eastern tropical Pacific.ACKNOWLEDGEMENTSWe thank Rob Dunbar, Jerry Wellington, and Mitch Colgan for sharing their monstrous coral withus. We thank Xiaomei Xu, Shuhui Zheng, and Sue Trumbore for shared equipment and support, PhilHowell for shared software, Tom Guilderson for helpful comments on the manuscript, MarkMcClure for editorial support, and Betty Leidel for her help with the Keck proposal. We thank theW M Keck Foundation for their generous support that allowed us to purchase and set up the KeckCarbon Cycle AMS Laboratory. We acknowledge the NSF Chemical Oceanography Program (toERMD), the UCOP Marine Sciences for support (to JH), and the Dreyfus Foundation for support (toERMD and TK).REFERENCESBjerknes J. 1969. Atmospheric teleconnections from theequatorial Pacific. Monthly Weather Review 97:163–72. Brown T, Farwell G, Grootes P, Schimidt F, Stuiver M.1993. Intra-annual variability of the radiocarbon con-tent of corals from the Galapagos Islands. Radiocar-bon 35(2):245–51.Druffel EM. 1981. Radiocarbon in annual coral ringsfrom the eastern tropical Pacific Ocean. GeophysicalResearch Letters 8:59–62.Druffel ERM. 1987. Bomb radiocarbon in the Pacific:annual and seasonal timescale variations. Journal ofMarine Research 45:667–98.Druffel ERM, Griffin S, Guilderson T, Kashgarian M,Schrag D. 2001. Changes of subtropical North Pacificradiocarbon and correlation with climate variability.Radiocarbon 43(1):15–25.Dunbar R, Wellington G, Colgan M, Glynn P. 1991. East-ern tropical Pacific corals monitor low latitude climateof the past 400 years. In: Betancourt J, Tharp V, edi-tors. Seventh Annual Pacific Climate (PACLIM)Workshop. California Department of Water Re-Variability of 14C in Corals from the Galapagos Islands 631sources. Interagency Ecological Studies Program.p 183–98.Grottoli A, Gille S, Druffel ERM, Dunbar R. 2003. Dec-adal timescale shift in the radiocarbon record of a cen-tral equatorial Pacific coral. Radiocarbon 45(1):91–9.Guilderson T, Schrag D. 1998. Abrupt shift in subsurfacetemperatures in the tropical Pacific associated withchanges in El Niño. Science 281:240–3.Guilderson T, Schrag D, Goddard E, Kashgarian M,Wellington G, Linsley B. 2000. Southwest subtropicalPacific surface water radiocarbon in a high-resolutioncoral record. Radiocarbon 42(2):249–56.Guilderson T, Schrag D, Kashgarian M, Southon J. 1998.Radiocarbon variability in the Western Equatorial Pa-cific inferred from a high-resolution coral record fromNauru Island. Journal of Geophysical Research103(C11):24,641–51.Richards AF. 1957. Volcanism in eastern Pacific Oceanbasin, 1954–1955. International Geological Con-gress, Mexico, Proceedings X 1:19–31.Stuiver M, Polach HA. 1977. Discussion: reporting of14C data. Radiocarbon 19(3):355–63.Toggweiler JR, Dixon K, Broecker WS. 1991. The Peruupwelling and the ventilation of the South Pacificthermocline. Journal Geophysical Research 96:20,467–97.Vogel JS, Southon JR, Nelson DE. 1987. Catalyst andbinder effects in the use of filamentous graphite forAMS. Nuclear Instruments and Methods in PhysicsResearch B 29:50–6.

English

Variability of radiocarbon during the 1760s in monthly corals from the Galapagos Islands

Radiocarbon (∆14C) measurements of monthly samples from a Galapagos surface coral are among the first data sets from the new Keck Carbon Cycle Accelerator Mass Spectrometry laboratory at the University of California, Irvine. An average ∆14C value of -62‰ is obtained for 144 measurements of samples from monthly coral bands that lived from about AD 1760-1771 (±6 yr). High ∆14C values were found during January through March, when upwelling was weak or absent at the Galapagos Islands. Low ∆14C values were obtained mid-year during strong upwelling. The average seasonal variability of ∆14C was 15-25‰, which is greater than that at other tropical and subtropical locations in the Pacific Ocean because of intense seasonal upwelling at this site. Periods of sustained high ∆14C values were found during 1762-1763 and 1766. A spectral analysis revealed that the spectral density for the ∆14C data displays most of its variance at the 5-yr cycle, which is reflective of El Nio periodicity during the 20th century

Druffel, Ellen R M

Griffin, Sheila

Hwang, Jeomshik

Komada, Tomoko

Beaupre, Steven R

Druffel-Rodriguez, Kevin C

Santos, Guaciara

Southon, John R

UC IrvineFaculty PublicationsTitleVariability of radiocarbon during the 1760s in monthly corals from the Galapagos IslandPermalinkhttps://escholarship.org/uc/item/3q42f770JournalRadiocarbon, 46(2)AuthorsDruffel, Ellen R MGriffin, SheilaHwang, Jeomshiket al.Publication Date2004Copyright InformationThis work is made available under the terms of a Creative Commons Attribution License, availalbe at https://creativecommons.org/licenses/by/4.0/ Peer reviewedeScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaRADIOCARBON, Vol 46, Nr 2, 2004, p 627–631  © 2004 by the Arizona Board of Regents on behalf of the University of Arizona© 2004 by the Arizona Board of Regents on behalf of the University of ArizonaProceedings of the 18th International Radiocarbon Conference, edited by N Beavan Athfield and R J SparksRADIOCARBON, Vol 46, Nr 2, 2004, p 627–631627VARIABILITY OF MONTHLY RADIOCARBON DURING THE 1760s IN CORALS FROM THE GALAPAGOS ISLANDSEllen R M Druffel1 • Sheila Griffin • Jeomshik Hwang • Tomoko Komada • Steven R Beaupre • Kevin C Druffel-Rodriguez • Guaciara M Santos • John SouthonDepartment of Earth System Science, University of California, Irvine, California 92697, USA.ABSTRACT. Radiocarbon (∆14C) measurements of monthly samples from a Galapagos surface coral are among the firstdata sets from the new Keck Carbon Cycle Accelerator Mass Spectrometry laboratory at the University of California, Irvine.An average ∆14C value of –62‰ is obtained for 144 measurements of samples from monthly coral bands that lived fromabout AD 1760–1771 (±6 yr). High ∆14C values were found during January through March, when upwelling was weak orabsent at the Galapagos Islands. Low ∆14C values were obtained mid-year during strong upwelling. The average seasonalvariability of ∆14C was 15–25‰, which is greater than that at other tropical and subtropical locations in the Pacific Oceanbecause of intense seasonal upwelling at this site. Periods of sustained high ∆14C values were found during 1762–1763 and1766. A spectral analysis revealed that the spectral density for the ∆14C data displays most of its variance at the 5-yr cycle,which is reflective of El Niño periodicity during the 20th century. INTRODUCTIONData from banded corals show that the radiocarbon (14C) “age” of pre-bomb, non-polar surfaceocean ranges from 620 14C yr (–72‰) in the eastern tropical Pacific to 340 14C yr (–40‰) in themid-gyre regions of the Pacific Ocean (Druffel 1987; Druffel et al. 2001). These non-contemporaryreservoir ages of surface dissolved inorganic carbon (DIC) are due mainly to vertical and advectivemixing of subsurface water having even “older” 14C ages because of radioactive decay during isola-tion from the atmosphere. Because 14C has a long turnover time (10 yr) in the atmosphere withrespect to the transfer to the sea surface, ∆14C of DIC in surface seawater is a tracer of circulationpatterns and thermocline depth. Records of upper ocean circulation changes during the past areessential for understanding present and future climate changes. Corals reflect the ∆14C in surface DIC and provide time histories of ∆14C on timescales of monthsto millennia. Records of ∆14C in the surface ocean reveal past variability of surface-subsurfacemixing and horizontal current shifts. We report monthly ∆14C measurements from a Galapagos coralrepresenting the 11-yr period AD 1760–1771 (±6 yr). We show that ∆14C is high during thebeginning of the year, when trade winds and upwelling were weak, and low ∆14C values during therest of the year, when wind-driven upwelling was strong.OCEANOGRAPHIC SETTING AND SAMPLE COLLECTIONThe coral sequence was collected by Drs Rob Dunbar, Mitch Colgan, and Gerard Wellington froma dead, massive head of Pavona clavus in Urvina Bay on the west coast of Isabela Island (0°15′S,91°22′W) in the Galapagos archipelago in 1986 (see Dunbar et al. 1991 for details). The coral sitewas directly in the path of the cold, nutrient-rich Peru Current and the Equatorial Counter Current(Cromwell Current), which brings subsurface water eastward. The coral and the shoreline of thecentral coast of Urvina Bay was raised above sea level in March 1954 because of the intrusion andupward movement of magma below the surface (Richards 1957). 1Corresponding author. Email: edruffel@uci.edu.628 E R M Druffel et al.METHODSDunbar et al. (1991) cleaned, X-rayed, and developed an age model for this coral sequence usinghigh-density bands that accrete from about January to April of each year. Based on preliminary230Th mass spectrometric analyses performed by Bruno Hamelin (LDEO), Dunbar et al. (1991) esti-mated an uncertainty of ±10 yr for ages at the base of the section (AD 1583), which translates to±6 yr for our samples during the 1760s (assuming the uncertainty is linear).  We performed ∆14C measurements on coral samples that had been ground off with a diamond-tipdremel tool to a width of 1 mm each from a vertical strip of coral (10 mm wide and 7 mm thick). Thesequence was 144 mm long and spanned the time period from late 1760 to mid-1771; the averagesampling frequency was 13 samples/yr.Sub-samples (8 mg each) were converted to CO2 in evacuated Vacutainers® with 85% phosphoricacid, then to graphite on Co catalyst with H2 gas as the reducing agent (Vogel et al. 1987). All of thesamples were analyzed for 14C/12C using accelerator mass spectrometry (AMS) at the Keck CarbonCycle AMS Laboratory at UC Irvine. Background subtraction was applied using 14C-free spar cal-cite. The ∆14C measurements had a total uncertainty (counting statistics and laboratory reproducibil-ity) of ±4–5‰ (1 σ). Radiocarbon measurements are reported as ∆14C values (for geochemical sam-ples with known age) according to Stuiver and Polach (1977). The ∆14C results were corrected usingδ13C values measured either by the AMS or on CO2 splits prior to graphitization.RESULTSThe average of 144 monthly ∆14C measurements (duplicate analyses and splits of the same CO2were averaged to a single point) from the Urvina Bay coral is –62 ± 6 (sd)‰. The range of the indi-vidual values is large (33‰), from –80‰ in early 1765 to –47‰ in mid-1770 (Figure 1). A least-squares fit of all monthly ∆14C measurements reveals no significant trend with time. Seasonal cycles are apparent in the ∆14C data during most years (Figure 1), though these cycles arenot well defined. The highest ∆14C values (large grey circles) are at the base of the high-densitybands (vertical lines in Figure 1), which are formed when surface waters are warm and upwelling isweak or absent (January through March) (Druffel 1981). The average of these 10 high ∆14C valuesis –53 ± 3‰. Most of the lowest ∆14C values (large black circles) were from samples in the middleof the bands. The average of the 11 low ∆14C values is –71 ± 5‰. A least-squares fit of the low ∆14Cvalues reveals a 6‰ decrease over the 11-yr time period, that for the high ∆14C values is notsignificant.DISCUSSIONThere appears to be an interannual fluctuation of the ∆14C values in this record. The ∆14C values for1765 and 1769 were lower than those for other years. Six to 7 consecutive high ∆14C values werefound for 1766 and 1770 (average –54‰ for both). A smoothed line of the monthly data is shown inFigure 1 using a Stineman function and suggests that, based on ∆14C data alone, El Niño eventsoccurred during 1762–1763, 1766, and possibly 1770.  During 1762–63, ∆14C values increased dur-ing the upwelling season relative to other years, similar to Guilderson and Schrag’s (1998) ∆14Cincrease in Galapagos coral after the climate shift of 1976.Periodicities within the monthly ∆14C record were investigated by spectral analysis using Spectralsoftware provided by Phil Howell (personal communication, 2000). The spectral density for the∆14C data displays a small amount of variance at the 1-yr cycle and most of its variance at the 5-yrVariability of 14C in Corals from the Galapagos Islands 629cycle (significant to the 95% confidence level). The 5-yr cycle, however, is close to the nyquist fre-quency of 5.5 yr (11 yr total record/2), so the significance of this result is questionable. Nonetheless,a 5-yr cycle is similar to the frequency of El Niño events during the 20th century, which would sug-gest that, within the limits of this short data set, El Niño (high ∆14C) events during the 1760s werenot different in frequency from those that were observed during recent times.  The average seasonal amplitude of ∆14C values for the Galapagos coral during the 1760s was 15 to25‰ (Figure 1), whereas those reported for recent corals from other Pacific sites are significantlylower (3–4‰ for Hawaiian coral, Druffel et al. 2001; 10–15‰ for Raratonga coral, Guilderson et al.2000; 10–15‰ for Fanning Island coral, Grottoli et al. 2003). Seasonal upwelling at the easterntropical Pacific is strong during the months of July through September, when the southeast tradewinds are strongest. During this period, cold (18–20 °C), nutrient-rich water upwells into the PeruCurrent and Equatorial Counter Current, both of which lave the Galapagos Islands. The lowest pre-bomb ∆14C values of any non-polar surface location are found at the Galapagos (Druffel 1981;Toggweiler et al. 1991) because of its proximity to intense equatorial and coastal upwelling.A comparison of average pre-bomb ∆14C values for corals from several Pacific locations revealsthat the monthly Galapagos record for the 1760s (–62 ± 6‰) and the annual Galapagos record for1930–1941 (Druffel 1989; –70 ± 6‰) were the lowest. Monthly samples from a Fanning Islandcoral (4°N, 159°W) have an average ∆14C value of –55 ± 5‰ (n = 217) between 1922 and 1947(Grottoli et al. 2003). In the western Pacific, Guilderson et al. (1998) determined the average ofmonthly ∆14C values at Nauru (1°S, 166°E) was –58‰ between 1947 and 1956. An average of allHawaiian bimonthly ∆14C analyses for the time period 1920.8–1923.0 was –46.4 ± 2.3‰ (DruffelFigure 1 ∆14C measurements of monthly coral bands from Urvina Bay, Galapagos, during the period 1760 to 1771. Theuncertainty of the age assignments in this age range is ±6 yr. Duplicate analyses of separate aliquots from the same coralsample and splits of the same CO2 were averaged to a single point. Large grey points represent the highest ∆14C valuesfor each year (lowest upwelling) and large black points represent the lowest ∆14C values for each year (most intenseupwelling). Vertical lines mark the base of the high-density bands, which are generally formed when upwelling is low.Horizontal lines (grey) are the least-squares fits of the high and low 14C values, and that (black) for all of the monthlyresults. The smoothed line of the monthly data is obtained using a Stineman function, which suggests that El Niño eventsoccurred during 1762–63 and possibly 1766. -90-80-70-60-50-400 20 40 60 80 100 120 140 ²14Cmm growth1762 1764 1770176817662 sigmaerrorEl Niño El Niño El Niño∆14 C630 E R M Druffel et al.et al. 2001). A coral from the subtropical South Pacific (Rarotonga, 21°S, 160°W) had a pre-bomb,average ∆14C value of –52‰ for the 1950s (Guilderson et al. 2000). These sites have higher ∆14Clevels because their mixed layers are more stable and there is little or no upwelling compared to theintense upwelling conditions in the eastern equatorial Pacific (Toggweiler et al. 1991).The physical mechanism that causes temporal variability of climate and ∆14C on an interannualtimescale is El Niño/Southern Oscillation (ENSO). ENSO is caused by interaction of the tropicalocean and atmosphere (Bjerknes 1969). 14C has been shown to increase in Galapagos coral bands thatgrew during El Niño events of the past 40 yr because upwelled waters no longer reached the sea sur-face (Druffel 1981; Brown et al. 1993), especially during mid-year (Guilderson and Schrag 1998).These events were generally coincident with warm waters that laved the region. Therefore, in thecase of El Niño events, high ∆14C measurements reflect a cessation of local upwelling, the presenceof high 14C surface water advected eastward along the Equator from the western warm pool, andtransport of off-equatorial water masses due to the southward migration of the ITCZ during El Niñoevents. We suggest that these conditions prevailed during 1762–63, 1766, and part of 1770. CONCLUSIONSRadiocarbon (∆14C) measurements of monthly samples from a Galapagos surface coral show anaverage ∆14C value of –62‰ for samples that lived during ~1760–1771. Periods of sustained high∆14C values were found for the years 1762–63 and possibly 1770. Spectral analysis of the ∆14C datashows that the spectral density has most of its variance in the 5-yr cycle, reflecting the El Niño cycle;some variance is seen for the annual cycle. The 5-yr cycle is similar to the frequency of El Niñoevents during the 20th century, which would indicate that El Niño (high ∆14C) events during the1760s occurred at a frequency similar to that during the last 100 yr. The seasonal variability of ∆14Cduring the 1760s, though not well defined, had a higher amplitude range than at other sites in thePacific because of the strong seasonal upwelling that is unique to the eastern tropical Pacific.ACKNOWLEDGEMENTSWe thank Rob Dunbar, Jerry Wellington, and Mitch Colgan for sharing their monstrous coral withus. We thank Xiaomei Xu, Shuhui Zheng, and Sue Trumbore for shared equipment and support, PhilHowell for shared software, Tom Guilderson for helpful comments on the manuscript, MarkMcClure for editorial support, and Betty Leidel for her help with the Keck proposal. We thank theW M Keck Foundation for their generous support that allowed us to purchase and set up the KeckCarbon Cycle AMS Laboratory. We acknowledge the NSF Chemical Oceanography Program (toERMD), the UCOP Marine Sciences for support (to JH), and the Dreyfus Foundation for support (toERMD and TK).REFERENCESBjerknes J. 1969. Atmospheric teleconnections from theequatorial Pacific. Monthly Weather Review 97:163–72. Brown T, Farwell G, Grootes P, Schimidt F, Stuiver M.1993. Intra-annual variability of the radiocarbon con-tent of corals from the Galapagos Islands. Radiocar-bon 35(2):245–51.Druffel EM. 1981. Radiocarbon in annual coral ringsfrom the eastern tropical Pacific Ocean. GeophysicalResearch Letters 8:59–62.Druffel ERM. 1987. Bomb radiocarbon in the Pacific:annual and seasonal timescale variations. Journal ofMarine Research 45:667–98.Druffel ERM, Griffin S, Guilderson T, Kashgarian M,Schrag D. 2001. Changes of subtropical North Pacificradiocarbon and correlation with climate variability.Radiocarbon 43(1):15–25.Dunbar R, Wellington G, Colgan M, Glynn P. 1991. East-ern tropical Pacific corals monitor low latitude climateof the past 400 years. In: Betancourt J, Tharp V, edi-tors. Seventh Annual Pacific Climate (PACLIM)Workshop. California Department of Water Re-Variability of 14C in Corals from the Galapagos Islands 631sources. Interagency Ecological Studies Program.p 183–98.Grottoli A, Gille S, Druffel ERM, Dunbar R. 2003. Dec-adal timescale shift in the radiocarbon record of a cen-tral equatorial Pacific coral. Radiocarbon 45(1):91–9.Guilderson T, Schrag D. 1998. Abrupt shift in subsurfacetemperatures in the tropical Pacific associated withchanges in El Niño. Science 281:240–3.Guilderson T, Schrag D, Goddard E, Kashgarian M,Wellington G, Linsley B. 2000. Southwest subtropicalPacific surface water radiocarbon in a high-resolutioncoral record. Radiocarbon 42(2):249–56.Guilderson T, Schrag D, Kashgarian M, Southon J. 1998.Radiocarbon variability in the Western Equatorial Pa-cific inferred from a high-resolution coral record fromNauru Island. 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Variability of radiocarbon during the 1760s in monthly corals from the Galapagos Island

Variability of radiocarbon during the 1760s in monthly corals from the Galapagos Islands

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