Seasonal radiocarbon and oxygen isotopes in a Galapagos coral: Calibration with climate indices

PUBLICATIONS Geophysical Research Letters RESEARCH LETTER 10.1002/2014GL060504 Key Points: • C and O in seasonal Galapagos corals are inversely correlated • Isotopes are correlated with ENSO and PDO in the east equatorial Paciﬁc • There are two sources of low C waters that upwell at the Galapagos Supporting Information: • Readme • Texts S1–S3 • Figure S1 • Table S1 Correspondence to: E. R. M. Druffel, edruffel@uci.edu Citation: Druffel, E. R. M., S. Grifﬁn, D. S. Glynn, R. B. Dunbar, D. A. Mucciarone, and J. R. Toggweiler (2014), Seasonal radiocarbon and oxygen isotopes in a Galapagos coral: Calibration with climate indices, Geophys. Res. Lett., 41, doi:10.1002/2014GL060504. Received 12 MAY 2014 Accepted 8 JUL 2014 Accepted article online 11 JUL 2014 Seasonal radiocarbon and oxygen isotopes in a Galapagos coral: Calibration with climate indices Ellen R. M. Druffel 1 , Sheila Grifﬁn 1 , Danielle S. Glynn 1 , Robert B. Dunbar 2 , David A. Mucciarone 2 , and J. Robert Toggweiler 3 Department of Earth System Science, University of California, Irvine, California, USA, 2 Environmental Earth System Science, Stanford University, Stanford, California, USA, 3 Geophysical Fluid Dynamics Laboratory/National Oceanic and Atmospheric Administration, Princeton University, Princeton, New Jersey, USA Abstract We present seasonal Δ 14 C and δ 18 O measurements from a Galapagos coral sequence that grew during the early 20th century. Our results show that both Δ 14 C and δ 18 O values are correlated with sea surface temperature in the Nino 3.4 region and are indicators of El Nino–Southern Oscillation. There is a signiﬁcant inverse correlation between Δ 14 C and δ 18 O values when Δ 14 C is lagged by ~2 months, indicating that sea surface temperature changes precede upwelling changes at this eastern equatorial location. We ﬁnd that cold season low-Δ 14 C values were higher after the Paciﬁc Decadal Oscillation (PDO) changed from a positive to a negative phase. Cold season high-δ 18 O values were signiﬁcantly higher after the PDO shift as well. These ﬁndings suggest that there are two sources of low-Δ 14 C waters that upwell at the Galapagos, Subantarctic Mode Water and shallow overturning water from the subpolar North Paciﬁc. 1. Introduction Radiocarbon ( 14 C) is naturally produced in the stratosphere and is present in dissolved inorganic carbon (DIC) in seawater. The Δ 14 C (per mil deviation from the 14 C/ 12 C ratio in 19th century wood/atmospheric CO 2 ) values in seawater DIC are highest in the surface and lowest in the deep ocean, because 14 C is radioactive and decays when it is isolated from its source. This decrease of Δ 14 C values with depth makes it a useful tracer of upwelling strength and climate events such as El Nino–Southern Oscillation (ENSO). When upwelling is suppressed during El Nino events, the Δ 14 C values in surface waters and annually banded corals of the east equatorial Paciﬁc (EEP) increase [Brown et al., 1993; Druffel et al., 2007; Guilderson and Schrag, 1998]. An annual Δ 14 C record obtained from a 360 year coral sequence from Urvina Bay, Galapagos Islands, was reported earlier [Druffel et al., 2007]. They attributed interannual and interdecadal variability to changes in ocean circulation, e.g., upwelling strength and the source of upwelled water from the subantarctic convergence zone. Rodgers et al. [2004] showed that Δ 14 C serves as a thermocline proxy in the EEP. The present work focuses on a seasonal Δ 14 C record of a portion of this coral to determine the relationship between Δ 14 C and δ 18 O (primarily recording sea surface temperature in this region) [Dunbar et al., 1994] and major climate shifts, e.g., Paciﬁc Decadal Oscillation (PDO) and ENSO. We report seasonal Δ 14 C values for the period 1939–1954 and seasonal δ 18 O values from 1943 to 1954. We ﬁnd that Δ 14 C and δ 18 O values are inversely correlated and that Δ 14 C values lag δ 18 O values by ~2 months. We ﬁnd that cold season low Δ 14 C values shift toward slightly higher values after 1947, coincident with the climate shift of the PDO to a negative phase. 2. Methods The coral used in this study (UR-86) was the same specimen of Pavona clavus that was used in previous studies of annual stable isotope [Dunbar et al., 1994] and radiocarbon [Druffel et al., 2007] measurements over the last 4 centuries. The coral was collected in 1986 from an uplifted reef in Urvina Bay on the west coast of Isabella Island (0°15′S, 91°22′W) in the Galapagos [Dunbar et al., 1994]. The coral section used was pristine aragonite. Our isotopic measurements were performed on seasonal samples that had been drilled at 1 mm spacing (6–13 samples/yr) from the top 15 (Δ 14 C) and 11 (δ 18 O) annual bands with a Dremel tool and diamond bit. DRUFFEL ET AL. ©2014. American Geophysical Union. All Rights Reserved.