J. Warren Beck

IntCal04 terrestrial radiocarbon age calibration, 0-26 cal kyr BP.

A new calibration curve for the conversion of radiocarbon ages to calibrated (cal) ages has been constructed and internationally ratified to replace IntCal98, which extended from 0-24 cal kyr BP (Before Present, 0 cal BP = AD 1950). The new calibration data set for terrestrial samples extends from 0-26 cal kyr BP, but with much higher resolution beyond 11.4 cal kyr BP than IntCal98. Dendrochronologically-dated tree-ring samples cover the period from 0-12.4 cal kyr BP. Beyond the end of the tree rings, data from marine records (corals and foraminifera) are converted to the atmospheric equivalent with a site-specific marine reservoir correction to provide terrestrial calibration from 12.4-26.0 cal kyr BP. A substantial enhancement relative to IntCal98 is the introduction of a coherent statistical approach based on a random walk model, which takes into account the uncertainty in both the calendar age and the 14C age to calculate the underlying calibration curve (Buck and Blackwell, this issue). The tree-ring data sets, sources of uncertainty, and regional offsets are discussed here. The marine data sets and calibration curve for marine samples from the surface mixed layer (Marine04) are discussed in brief, but details are presented in Hughen et al. (this issue a). We do not make a recommendation for calibration beyond 26 cal kyr BP at this time; however, potential calibration data sets are compared in another paper (van der Plicht et al., this issue).

Marine04 marine radiocarbon age calibration, 0-26 cal kyr BP

New radiocarbon calibration curves, IntCal04 and Marine04, have been constructed and internationally rati- fied to replace the terrestrial and marine components of IntCal98. The new calibration data sets extend an additional 2000 yr, from 0-26 cal kyr BP (Before Present, 0 cal BP = AD 1950), and provide much higher resolution, greater precision, and more detailed structure than IntCal98. For the Marine04 curve, dendrochronologically-dated tree-ring samples, converted with a box diffusion model to marine mixed-layer ages, cover the period from 0-10.5 cal kyr BP. Beyond 10.5 cal kyr BP, high-res- olution marine data become available from foraminifera in varved sediments and U/Th-dated corals. The marine records are corrected with site-specific 14C reservoir age information to provide a single global marine mixed-layer calibration from 10.5-26.0 cal kyr BP. A substantial enhancement relative to IntCal98 is the introduction of a random walk model, which takes into account the uncertainty in both the calendar age and the 14C age to calculate the underlying calibration curve (Buck and Blackwell, this issue). The marine data sets and calibration curve for marine samples from the surface mixed layer (Marine04) are discussed here. The tree-ring data sets, sources of uncertainty, and regional offsets are presented in detail in a companion paper by Reimer et al. (this issue). ABSTRACT. New radiocarbon calibration curves, IntCal04 and Marine04, have been constructed and internationally rati- fied to replace the terrestrial and marine components of IntCal98. The new calibration data sets extend an additional 2000 yr, from 0-26 cal kyr BP (Before Present, 0 cal BP = AD 1950), and provide much higher resolution, greater precision, and more detailed structure than IntCal98. For the Marine04 curve, dendrochronologically-dated tree-ring samples, converted with a box diffusion model to marine mixed-layer ages, cover the period from 0-10.5 cal kyr BP. Beyond 10.5 cal kyr BP, high-res- olution marine data become available from foraminifera in varved sediments and U/Th-dated corals. The marine records are corrected with site-specific 14C reservoir age information to provide a single global marine mixed-layer calibration from 10.5-26.0 cal kyr BP. A substantial enhancement relative to IntCal98 is the introduction of a random walk model, which takes into account the uncertainty in both the calendar age and the 14C age to calculate the underlying calibration curve (Buck and Blackwell, this issue). The marine data sets and calibration curve for marine samples from the surface mixed layer (Marine04) are discussed here. The tree-ring data sets, sources of uncertainty, and regional offsets are presented in detail in a companion paper by Reimer et al. (this issue).

Interdecadal variation in the extent of South Pacific tropical waters during the Younger Dryas event

During the Younger Dryas event, about 12,000 years ago, the Northern Hemisphere cooled by between 2 and 10 °C (refs 1, 2) whereas East Antarctica experienced warming. But the spatial signature of the event in the southern mid-latitudes and tropics is less well known, as records are sparse and inconclusive. Here we present high-resolution analyses of skeletal Sr/Ca and 18O/16O ratios for a giant fossil Diploastrea heliopora coral that was preserved in growth position on the raised reef terraces of Espiritu Santo Island, Vanuatu, in the southwestern tropical Pacific Ocean. Our data indicate that sea surface temperatures in Vanuatu were on average 4.5 ± 1.3 °C cooler during the Younger Dryas event than today, with a significant interdecadal modulation. The amplified annual cycle of sea surface temperatures, relative to today, indicates that cooling was caused by the compression of tropical waters towards the Equator. The positive correlation in our record between the oxygen isotope ratios of sea water and sea surface temperatures suggests that the South Pacific convergence zone, which brings 18O-depleted precipitation to the area today, was not active during the Younger Dryas period.

A high-resolution radiocarbon calibration between 11,700 and 12,400 calendar years BP derived from 230Th ages of corals from Espiritu Santo Island, Vanuatu

This paper presents radiocarbon results from a single Diploastrea heliopora coral from Vanuatu that lived during the Younger Dryas climatic episode, between ca. 11,700 and 12,400 calendar yr BP. The specimen has been independently dated with multiple 230Th measurements to permit calibration of the 14C time scale. Growth bands in the coral were used to identify individual years of growth. 14C measurements were made on each year. These values were averaged to achieve decadal resolution for the 14C calibration. The relative uncertainty of the decadal 14C data was below 1% (2σ). The data are in good agreement with the existing dendrochronology and allow for high-resolution calibration for most years. Variations in the fine structure of the 14C time series preserved in this specimen demonstrate sporadic rapid increases in the Δ14C content of the surface ocean and atmosphere. Certain sharp rises in Δ14C are coincident with gaps in coral growth evidenced by several hiatuses. These may be related to rapid climatic changes that occurred during the Younger Dryas. This is the first coral calibration with decadal resolution and the only such data set to extend beyond the dendrochronology-based 14C calibration.

Terrestrial evidence for a spatial structure of tropical-polar interconnections during the Younger Dryas episode

The Younger Dryas chronozone, recognised in northern high-latitude areas as a cold event between 11 000 and 10 000 14C yr BP (12 900–11 600 cal. yr BP), seems to manifest itself globally in different ways. Here, we examine well-dated stratigraphic sequences together with high-resolution proxy data plots from sites across our study area, the arid–semi-arid transition zone in northern China. This climatically sensitive area of China records a cold, dry Younger Dryas climate which was punctuated by a brief period of summer monsoon precipitation. We have since found that similar climatic sequences have been reported from the Sahel and the equatorial region of Africa. Based on evidence from these sites, together with other published data, we postulate that precipitation during the Younger Dryas chronozone was indicative of a low-latitude driving force superimposed on the high-latitude cold background. This rain belt rearrangement was most probably caused by an interaction between cold air advection and summer moisture transport across the tropical Pacific Ocean. Examination of high-resolution proxies suggests short-term climate fluctuations indicative of a global teleconnection involving moist air transportation patterns from the tropics to higher latitudes, varying with ENSO and other tropical factors.

Disentangling Geomagnetic and Precipitation Signals in an 80-kyr Chinese Loess Record of 10Be

The cosmogenic radionuclide 10 Be is produced by cosmic-ray spallation in Earths atmosphere. Its production rate is regulated by the geomagnetic field intensity, so that its accumulation rate in aeolian sediments can, in principle, be used to derive high-resolution records of geomagnetic field changes. However, 10 Be atmospheric fallout rate also varies locally depending on rainfall rate. The accumulation rate of 10 Be in sediments is further complicated by overprinting of the geomagnetic and precipitation signals by 10 Be attached to remobilized dust, which fell from the atmosphere at some time in the past. Here, we demonstrate that these signals can be deconvoluted to derive both geomagnetic field intensity and paleoprecipitation records of Asian Monsoon intensity in an 80,000-yr-long 10 Be record from Chinese loess. The strong similarity between our derived paleomagnetic intensity record and the SINT 200 (Guyodo and Valet 1996) and NAPIS 75 (Laj et al. 2002) stacked-marine records suggests that this method might be used to produce multimillion-yr-long records of paleomagnetic intensity from loess. This technique also reveals a new method for extracting quantitative paleoprecipitation records from continental interior regions. Our derived precipitation record is broadly similar to the speleothem δ 18 O-based records of paleo-Asian Monsoon intensity from Dongge (Yuan et al. 2004) and Hulu (Wang et al. 2001) caves, and suggests that the paleo-Asian Monsoon intensity may be responding to a combination of both Northern and Southern Hemisphere insolation forcing.

PRELIMINARY REPORT OF THE FIRST WORKSHOP OF THE INTCAL04 RADIOCARBON CALIBRATION/COMPARISON WORKING GROUP

The first meeting of the IntCal04 working group took place at Queens University Belfast from April 15 to 17, 2002. The participants are listed as co-authors of this report. The meeting considered criteria for the acceptance of data into the next official calibration dataset, the importance of including reliable estimates of uncertainty in both the radiocarbon ages and the cal ages, and potential methods for combining datasets. This preliminary report summarizes the criteria that were dis- cussed, but does not yet give specific recommendations for inclusion or exclusion of individual datasets.

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

Timing of the Brunhes-Matuyama magnetic polarity reversal in Chinese loess using 10Be

In Chinese loess, the Brunhes-Matuyama (B-M) geomagnetic reversal occurs ∼25 k.y. prior to the age found in marine sediments. This offset has been attributed by some to post-depositional magnetic overprinting of loess, while others have argued it is due to errors in the loess time scale. Here we solve this long-standing debate by exploiting a new method to extract reproducible records of geomagnetic field intensity from loess with 10 Be—a proxy for global average geomagnetic field intensity—and using it to show that a pronounced minimum in field intensity (a requirement for dipole field reversal) is recorded in two separate loess records at ca. 780 ± 3 kyr B.P. This timing is synchronous with the B-M reversal timing seen in marine records, verifying the standard loess time scale as correct, but it is ∼25 k.y. younger than the age (depth) of the magnetic polarity reversal recorded in these same Chinese loess sediments, demonstrating that loess magnetic overprinting has occurred.

Reconstruction of 130-kyr Relative Geomagnetic Intensities from 10Be in Two Chinese Loess Sections

Efforts to extract weak geomagnetic excursion signals from Chinese loess-paleosol 10Be have generally been unsuccessful due to the complexities of its accumulation, because the geomagnetic and climate (precipitation and dust) signals contained in loess-paleosol sequence are tightly overprinted. Here, we present a reconstruction of geomagnetic relative paleointensities for the past 130 kyr from 10Be records in 2 Chinese loess-paleosol sections using a correction based on the correlation of 10Be with magnetic susceptibility (SUS) to remove the climatic contamination. Both these records reveal the Laschamp and Blake events, which lie in the loess and paleosol (L1SS1 and S1SS3) horizons corresponding to mid-MIS 3 and 5e, respectively. The good agreement between our results and other geomagnetic intensities reconstructions from Atlantic and Pacific sediments indicates that our method is robust. Our study suggests the potential application of loess-paleosol 10Be for reconstructing geomagnetic intensity variations spanning the whole Quaternary.