R. L. Edwards

A Large Drop in Atmospheric 14C/12C and Reduced Melting in the Younger Dryas, Documented with 230Th Ages of Corals.

Paired carbon-14 (14C) and thorium-230(230Th) ages were determined on fossil corals from the Huon Peninsula, Papua New Guinea. The ages were used to calibrate part of the 14C time scale and to estimate rates of sea-level rise during the last deglaciation. An abrupt offset between the 14C and 230Th ages suggests that the atmospheric 14C/12C ratio dropped by 15 percent during the latter part of and after the Younger Dryas (YD). This prominent drop coincides with greatly reduced rates of sea-level rise. Reduction of melting because of cooler conditions during the YD may have caused an increase in the rate of ocean ventilation, which caused the atmospheric 14C/12C ratio to fall. The record of sea-level rise also shows that globally averaged rates of melting were relatively high at the beginning of the YD. Thus, these measurements satisfy one of the conditions required by the hypothesis that the diversion of meltwater from the Mississippi to the St. Lawrence River triggered the YD event.

The Last Glacial Termination

Warming Up For the past half-million years, our planet has passed through a cycle of glaciation and deglaciation every 100,000 years or so. Each of these cycles consists of a long and irregular period of cooling and ice sheet growth, followed by a termination—a period of rapid warming and ice sheet decay—that precedes a relatively short warm interval. But what causes glacial terminations? Denton et al. (p. 1652) review the field and propose a chain of events that may explain the hows and whys of Earths emergence from the last glacial period. Pulling together many threads from both hemispheres suggests a unified causal chain involving ice sheet volume, solar radiation energy, atmospheric carbon dioxide concentrations, sea ice, and prevailing wind patterns. A major puzzle of paleoclimatology is why, after a long interval of cooling climate, each late Quaternary ice age ended with a relatively short warming leg called a termination. We here offer a comprehensive hypothesis of how Earth emerged from the last global ice age. A prerequisite was the growth of very large Northern Hemisphere ice sheets, whose subsequent collapse created stadial conditions that disrupted global patterns of ocean and atmospheric circulation. The Southern Hemisphere westerlies shifted poleward during each northern stadial, producing pulses of ocean upwelling and warming that together accounted for much of the termination in the Southern Ocean and Antarctica. Rising atmospheric CO2 during southern upwelling pulses augmented warming during the last termination in both polar hemispheres.

Rapid sea-level fall and deep-ocean temperature change since the last interglacial period

We have dated Huon Peninsula, Papua New Guinea and Barbados corals that formed at times since the Last Interglacial Period, applying both 230 Th and 231 Pa dating techniques as a test of age accuracy. We show that Marine Isotope Stage (MIS) 5e ended prior to 113.1 8 0.7 kyr, when sea level was 319 m. During MIS 5b sea level was 357 m at 92.6 8 0.5 kyr, having dropped about 40 m in approximately 10 kyr during the MIS 5c^5b transition. Sea level then rose more than 40 m during the MIS 5b^5a transition, also in about 10 kyr. MIS 5a lasted until at least 76.2 8 0.4 kyr, at a level of 324 m at that time. Combined with earlier data that places MIS 4 sea level at 381 m at 70.8 kyr, our late MIS 5a data indicate that sea level fell almost 60 m in less than 6 kyr (10.6 m/kyr) during the MIS 5^4 transition. The magnitude of the drop is half that of the glacial^interglacial amplitude and approximatelyequivalent to the volume of the present-dayAntarctic Ice Sheet. During this interval the minimum average rate of net continental ice accumulation was 18 cm/yr, likely facilitated by efficient moisture transport from lower latitudes. At three specific times (60.6 8 0.3, 50.8 8 0.3, and 36.8+0.2 kyr) during MIS 3, sea level was between 385 and 374 m. Sea level then dropped to 3107 m at 23.7 8 0.1 kyr early in MIS 2, before dropping further to Last Glacial Maximum (LGM) values and then rising to present values during the last deglaciation. Times of rapid sea-level drop correspond to times of high winter insolation at low northern latitudes and high winter latitudinal gradients in northern hemisphere insolation, supporting the idea that these factors mayhave resulted in high water-vapor pressure in moisture sources and efficient moisture transport to high-latitude glaciers, therebycontributing to glacial buildup. We combined our sea-level results with deep-sea N 18 O records as a means of estimating the temperature and ice-volume components in the marine N 18 O record. This analysis confirms large deep-ocean temperature shifts following MIS 5e and during Termination I. Deep-ocean temperatures changed bymuch smaller amounts between MIS 5c and 2. Maximum temperature shift in the deep Pacific is about 2‡, whereas the shift at a site in the Atlantic is 4‡. Under glacial conditions temperatures at both sites are near the freezing point. The shift in the Atlantic is likelycaused bya combination of changing proportions of northern and southern source waters as well as changing temperature at the sites where these deep waters form.

Highly Variable El Niño–Southern Oscillation Throughout the Holocene

ENSO Variability The El Niño–Southern Oscillation (ENSO) is the most energetic, quasiperiodic climate oscillation in the world—every few years warming large expanses of the surface equatorial Pacific Ocean surface and impacting temperatures and rainfall patterns across the globe. A pressing question, in the context of global warming, is whether ENSO might be affected by the rising atmospheric temperatures caused by anthropogenic greenhouse gas emissions. Climate models do not agree on the answer to this question, but one place to look for data about how global temperatures might influence ENSO is the record of past ENSO variability. Cobb et al. (p. 67) present a record of ENSO variability spanning the past 7000 years, in an attempt better to define its response to insolation forcing over this same period. The findings reveal high variability in ENSO behavior that has no clear dependence on insolation, which implies that a link to warming, if it exists, may be difficult to detect. Coral records show that the El Niño–Southern Oscillation may be less sensitive to past climate forcing than previously thought. The El Niño–Southern Oscillation (ENSO) drives large changes in global climate patterns from year to year, yet its sensitivity to continued anthropogenic greenhouse forcing is uncertain. We analyzed fossil coral reconstructions of ENSO spanning the past 7000 years from the Northern Line Islands, located in the center of action for ENSO. The corals document highly variable ENSO activity, with no evidence for a systematic trend in ENSO variance, which is contrary to some models that exhibit a response to insolation forcing over this same period. Twentieth-century ENSO variance is significantly higher than average fossil coral ENSO variance but is not unprecedented. Our results suggest that forced changes in ENSO, whether natural or anthropogenic, may be difficult to detect against a background of large internal variability.

Timing and climatic impact of Greenland interstadials recorded in stalagmites from northern Turkey.

A 50 kyr-long exceptionally well-dated and highly resolved stalagmite oxygen (δ 18O) and carbon (δ 13C) isotope record from Sofular Cave in northwestern Turkey helps to further improve the dating of Greenland Interstadials (GI) 1, and 3–12. Timing of most GI in the Sofular record is consistent within ±10 to 300 years with the “iconic” Hulu Cave record. Larger divergences (>500 years) between Sofular and Hulu are only observed for GI 4 and 7. The Sofular record differs from the most recent NGRIP chronology by up to several centuries, whereas age offsets do not increase systematically with depth. The Sofular record also reveals a rapid and sensitive climate and ecosystem response in the eastern Mediterranean to GI, whereas a phase lag of ∼100 years between climate and full ecosystem response is evident. Finally, results of spectral analyses of the Sofular isotope records do not support a 1,470-year pacing of GI.

A +20 m middle Pleistocene sea-level highstand (Bermuda and the Bahamas) due to partial collapse of Antarctic ice

Marine deposits at +20 ± 3 m on the tectonically stable coastlines of Bermuda and the Bahamas support the hypothesis of a partial collapse of the Antarctic ice sheet during the middle Pleistocene. Beach sediments fill a sea cave at +22 m in Bermuda, and horizontal, fenestrae-filled beds crop out on platforms at two sites as high as +21 m in Eleuthera, Bahamas. Carbonate beach sands are bound by an early generation of isopachous fibrous cement that is characteristic of a phreatic marine environment. Amino acid racemization and TIMS (thermal-ionization mass spectrometry) dates constrain the age of the deposits to between 390 and 550 ka, while proxy evidence supports a correlation with oxygen isotope stage 11. This direct geologic evidence of a 20% decrease in polar ice during the middle Pleistocene has important implications for the stability of ice sheets during warm interglaciations.

Uranium-series Dating of Marine and Lacustrine Carbonates

Of the possible uranium-series dating schemes, the most important and most widely applied to marine carbonates is 230Th dating, with 231Pa dating playing an increasingly important role. For this reason, this review will focus on these two methods. 230Th dating, also referred to as U/Th dating or 238U-234U-230Th dating, involves calculating ages from radioactive decay and ingrowth relationships among 238U, 234U, and 230Th. 232Th is also typically measured as a long-lived, essentially stable index isotope (over the time scales relevant to 230Th dating). At present 230Th dating can, in principle, be used to date materials as young as 3 years and in excess of 600,000 years (Edwards et al. 1987a, 1993; Edwards 1988; see Stirling et al. 2001 for an example of dating corals in excess of 600,000 years old). 231Pa dating, also referred to as U/Pa dating, involves calculating ages from the ingrowth of 231Pa from its grandparent 235U. At present 231Pa dating can be used to date materials as young as 10 years and as old 250,000 years (Edwards et al. 1997). 230Th dating covers all of the 231Pa time range and more, with somewhat higher precision, and is therefore the method of choice if a single method is applied. However, the combination of 231Pa and 230Th dating is of great importance in assessing possible diagenetic mobilization of the pertinent nuclides, and thereby, the accuracy of the ages (Allegre 1964; Ku 1968). Even if the primary age exceeds the 250,000 year limit of 231Pa dating, the combined methods can be used to assess the degree to which the samples have remained closed over the past 250,000 years (e.g., Edwards et al. 1997). Thus …

Accelerated drawdown of meridional overturning in the late‐glacial Atlantic triggered by transient pre‐H event freshwater perturbation

[1] Abrupt decreases of the Atlantic meridional overturning circulation (MOC) during the Late Pleistocene have been directly linked to catastrophic discharges of glacimarine freshwater, triggering disruption of northward marine heat transport and causing global climate changes. Here we provide measurements of excess sedimentary 231 Pa/ 230 Th from a high-accumulation sediment drift deposit in the NE Atlantic that record a sequence of sudden variations in the rate of MOC, associated deep ocean ventilation and surface-ocean climatology. The data series reveal a sequential decrease in the MOC rate at � 18.0 ka BP ago that coincides with only transient and localized freshwater inputs. This change represents a substantial, though not total, cessation in MOC that predates the major Heinrich (H1) meltwater event by at least 1,200 years. These results highlight the potential of targeted freshwater perturbations in promoting substantial MOC changes without a direct linking with catastrophic freshwater surges. Citation: Hall, I. R., S. B. Moran, R. Zahn, P. C. Knutz, C.-C. Shen, and R. L. Edwards (2006), Accelerated drawdown of meridional overturning in the late-glacial Atlantic triggered by transient pre-H event freshwater perturbation, Geophys. Res. Lett., 33, L16616, doi:10.1029/2006GL026239.

Distribution of 230Th in the Labrador Sea and its relation to ventilation

Abstract Measurements of dissolved and particulate230Th and232Th by thermal ionization mass spectrometry were made on samples collected from the Labrador Sea and the Denmark Strait and Iceland-Scotland overflow waters. The large-scale feature of low and invariant230Th evident in deep waters of the Labrador Sea and previously observed in the deep northern Atlantic can be reproduced using a reversible scavenging model that includes the effect of ventilation. Advective transport also must be important in the redistribution of230Th in other regions of the Atlantic and for other long-lived tracers, such as231Pa,26Al and10Be.

Radiocarbon calibration and comparison to 50 kyr BP with paired 14C and 230Th dating of corals from Vanuatu and Papua New Guinea

We calibrated portions of the radiocarbon time scale with combined (super 230) Th, (super 231) Pa, (super 14) C measurements of corals collected from Espiritu Santo, Vanuatu and the Huon Peninsula, Papua New Guinea. The new data map (super 14) C variations ranging from the current limit of the tree-ring calibration (11,900 calendar years before present [cal BP], Kromer and Spurk 1998, now updated to 12,400 cal BP, see Kromer et al., this issue), to the (super 14) C-dating limit of 50,000 cal BP, with detailed structure between 14 to 16 cal kyr BP and 19 to 24 cal kyr BP. Samples older than 25,000 cal BP were analyzed with high-precision (super 231) Pa dating methods (Pickett et al. 1994; Edwards et al. 1997) as a rigorous second check on the accuracy of the (super 230) Th ages. These are the first coral calibration data to receive this additional check, adding confidence to the age data forming the older portion of the calibration. Our results, in general, show that the offset between calibrated and (super 14) C ages generally increases with age until about 28,000 cal BP, when the recorded (super 14) C age is nearly 6800 yr too young. The gap between ages before this time is less; at 50,000 cal BP, the recorded (super 14) C age is 4600 yr too young. Two major (super 14) C-age plateaus result from a 13 ppm drop in Delta (super 14) C between 14-15 cal kyr BP and a 700 ppm drop in Delta (super 14) C between 22-25 cal kyr BP. In addition, a large atmospheric Delta (super 14) C excursion to values over 1000 ppm occurs at 28 cal kyr BP. Between 20 and 10 cal kyr BP, a component of atmospheric Delta (super 14) C anti-correlates with Greenland ice Delta (super 18) O, indicating that some portion of the variability in atmospheric Delta (super 14) C is related to climate change, most likely through climate-related changes in the carbon cycle. Furthermore, the 28-kyr excursion occurs at about the time of significant climate shifts. Taken as a whole, our data indicate that in addition to a terrestrial magnetic field, factors related to climate change have affected the history of atmospheric (super 14) C.