Publications

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2016
Sanchez, SC, Charles CD, Carriquiry JD, Villaescusa JA.  2016.  Two centuries of coherent decadal climate variability across the Pacific North American region. Geophysical Research Letters. 43:9208-9216.   10.1002/2016gl069037   AbstractWebsite

The decadal variability of the Pacific Ocean and North American hydroclimate are subjects of immediate concern for society, yet the length of the instrumental record limits full mechanistic understanding of this variability. Here we introduce a 178year, seasonally resolved coral oxygen isotopic record from Clarion Island (18 degrees N, 115 degrees W), a sampling a subtropical region that is strongly influenced by the decadal-scale fluctuations of the North Pacific Gyre Oscillation and a region that serves as a critical locus for the communication of climate anomalies with the tropics. This Mexican Pacific coral record is highly correlated to coral records from the central tropical Pacific and tree ring records from western North America. Significant changes in the amplitude of oceanic decadal variability in the early nineteenth century are mirrored in the drought reconstructions in western North America. The spatial manifestation of this relationship was relatively invariant, despite notable changes in the climatic mean state.

2010
Herguera, JC, Herbert T, Kashgarian M, Charles C.  2010.  Intermediate and deep water mass distribution in the Pacific during the Last Glacial Maximum inferred from oxygen and carbon stable isotopes. Quaternary Science Reviews. 29:1228-1245.   10.1016/j.quascirev.2010.02.009   AbstractWebsite

Intermediate ocean circulation changes during the last Glacial Maximum (LGM) in the North Pacific have been linked with Northern Hemisphere climate through air sea interactions, although the extent and the source of the variability of the processes forcing these changes are still not well resolved. The ventilated volumes and ages in the upper wind driven layer are related to the wind stress curl and surface buoyancy fluxes at mid to high latitudes in the North Pacific. In contrast, the deeper thermohaline layers are more effectively ventilated by direct atmosphere-sea exchange during convective formation of Subantarctic Mode Waters (SAMW) and Antarctic Intermediate Waters (AAIW) in the Southern Ocean, the precursors of Pacific Intermediate Waters (PIW) in the North Pacific. Results reported here show a fundamental change in the carbon isotopic gradient between intermediate and deep waters during the LGM in the eastern North Pacific indicating a deepening of nutrient and carbon rich waters. These observations suggest changes in the source and nature of intermediate waters of Southern Ocean origin that feed PIW and enhanced ventilation processes in the North Pacific, further affecting paleoproductivity and export patters in this basin. Furthermore, oxygen isotopic results indicate these changes may have been accomplished in part by changes in circulation affecting the intermediate depths during the LGM. (C) 2010 Elsevier Ltd. All rights reserved.

2009
Nurhati, IS, Cobb KM, Charles CD, Dunbar RB.  2009.  Late 20th century warming and freshening in the central tropical Pacific. Geophysical Research Letters. 36   10.1029/2009gl040270   AbstractWebsite

Global climate models and analyses of instrumental datasets provide a wide range of scenarios for future tropical Pacific climate change, limiting the accuracy of regional climate projections. Coral records provide continuous reconstructions of tropical Pacific climate trends that are difficult to quantify using the short, sparse instrumental datasets available from the tropical Pacific. Here, we present coral-based reconstructions of late 20th century sea-surface temperature and salinity trends from several islands in the central tropical Pacific. The coral data reveal warming trends that increase towards the equator, implying a decrease in equatorial upwelling in the last decades. Seawater freshening trends on the southern edge of the Inter-Tropical Convergence Zone suggest a strengthening and/or an equatorward shift of the convergence zone. Together, the new coral records support a late 20th century trend towards "El Nino-like" conditions in the tropical Pacific, in line with the majority of coupled global climate model projections. Citation: Nurhati, I. S., K. M. Cobb, C. D. Charles, and R. B. Dunbar (2009), Late 20th century warming and freshening in the central tropical Pacific, Geophys. Res. Lett., 36, L21606, doi:10.1029/2009GL040270.

2006
Field, DB, Baumgartner TR, Charles CD, Ferreira-Bartrina V, Ohman MD.  2006.  Planktonic foraminifera of the California Current reflect 20th-century warming. Science. 311:63-66.   10.1126/science.1116220   AbstractWebsite

It is currently unclear whether observed pelagic ecosystem responses to ocean warming, such as a mid-1970s change in the eastern North Pacific, depart from typical ocean variability. We report variations in planktonic foraminifera from varved sediments off southern California spanning the past 1400 years. Increasing abundances of tropical/subtropical species throughout the 20th century reflect a warming trend superimposed on decadal-scale fluctuations. Decreasing abundances of temperate/subpolar species in the late 20th century indicate a deep, penetrative warming not observed in previous centuries. These results imply that 20th-century warming, apparently anthropogenic, has already affected lower trophic levels of the California Current.

2003
Charles, CD, Cobb KM, Moore MD, Fairbanks RG.  2003.  Monsoon-tropical ocean interaction in a network of coral records spanning the 20th century. Marine Geology. 201:207-222.   10.1016/s0025-3227(03)00217-2   AbstractWebsite

The 20th century evolution of basin-wide gradients in surface ocean properties provides one essential test for recent models of the interaction between the Asian monsoon and the tropical ocean, because various feedback mechanisms should result in characteristic regional patterns of variability. Although the instrumental record of climate variability in the tropics is essentially limited to the last few decades, the stable isotopic composition of living corals provides an effective means for extending the instrumental observations. Here we present two coral isotopic records from the Indonesian Maritime Continent, and we use these records with other previously published records to describe: (i) the relationship between western Pacific and central Pacific climate variability over the past century, with special emphasis on the biennial band; and (ii) the strength of the west-east 'Indian Ocean Dipole'. We find that the amplitude of the biennial cycle in the Pacific did not vary inversely with the strength of ENSO (El Nino Southern Oscillation), as might be expected from some models of monsoonal feedback on the central Pacific. Instead, the biennial variability was modulated on decadal timescales throughout much of the Pacific. We also show that the zonal oxygen isotopic gradient in the Indian Ocean coral records was significantly correlated with central Pacific sea surface temperature on a variety of timescales. Thus, it is likely that this 'coral dipole' was a product of strong ENSO-like teleconnections over the Indian Ocean, as opposed to being the result of unique Indian Ocean or monsoonal dynamics. (C) 2003 Elsevier B.V. All rights reserved.

Cobb, KM, Charles CD, Cheng H, Kastner M, Edwards RL.  2003.  U/Th-dating living and young fossil corals from the central tropical Pacific. Earth and Planetary Science Letters. 210:91-103.   10.1016/s0012-821x(03)00138-9   AbstractWebsite

This study evaluates the accuracy of U/Th dates for young (< a few thousand years old) reef corals, both living and fossil, and explores strategies for refining those dates. The high precision of the U/Th method (+/-1-2%) for dating young corals is well-established. Earlier studies have demonstrated the method's accuracy for select samples of known age. However, these studies have focused on typical samples that have extremely low Th-232 concentrations (tens of pg/ g). Here we study the dating systematics of young corals that have low but significant amounts of Th-232 (up to 1000 pg/g), indicating the presence of small fractions of non-radiogenic Th-230 (i.e. Th-230 not generated by in situ U decay). We report U/Th ages for living and subaerially exposed fossil corals from Palmyra Island, located in the central tropical Pacific, that range from 50 to 700 yr old. The Palmyra corals contain varying amounts of Th-232 and small fractions of associated non-radiogenic Th-230. Uncertainty associated with the correction for non-radiogenic Th-230 can lead to significant errors in U/Th dates. We have characterized non-radiogenic Th-230/Th-232 values, (Th-230/Th-232)(nr), as a means of minimizing this source of error. We calculate (Th-230/Th-232)(nr), values ranging from 0 to 2 X 10(-5) for the Palmyra living corals by comparing measured U/Th dates to absolute dates for the living coral, whose chronology is well-established. For the fossil corals, we employ three different approaches to arrive at (Th-230/Th-232)(nr) estimates. First, we compare measured U/Th dates to absolute dates in samples from a young fossil coral that overlaps the living coral. Next, we use the firm relative dating constraints imposed by five overlapping fossil corals from the 14th-15th centuries to calculate (Th-230/Th-232)(nr) values. Finally, we attempt to anchor the 14th-15th century floating coral chronology to an absolute chronology by correlating the climate signals in the coral records to those in absolutely dated climate proxy records. All lines of evidence point to a range of (Th-230/Th-232)(nr) for fossil corals that overlaps the range determined for the living coral, suggesting that most of the thorium is primary or is added while the coral is still alive. Our work also demonstrates the utility of multiple (Th-230/Th-232)(nr) estimates. Most importantly, we demonstrate a method by which accurate (+/- 5 yr) U/Th-based chronologies can be obtained for young fossil corals with significant Th-232 concentrations.

2002
Ninnemann, US, Charles CD.  2002.  Changes in the mode of Southern Ocean circulation over the last glacial cycle revealed by foraminiferal stable isotopic variability. Earth and Planetary Science Letters. 201:383-396.   10.1016/s0012-821x(02)00708-2   AbstractWebsite

Benthic foraminiferal oxygen and carbon isotopic records from Southern Ocean sediment cores show that during the last glacial period, the South Atlantic sector of the deep Southern Ocean filled to roughly 2500 m with water uniformly low in VC, resulting in the appearance of a strong mid-depth nutricline similar to those observed in glacial northern oceans. Concomitantly, deep water isotopic gradients developed between the Pacific and Atlantic sectors of the Southern Ocean; the delta(13)C of benthic foraminifera in Pacific sediments remained significantly higher than those in the Atlantic during the glacial episode. These two observations help to define the extent of what has become known as the 'Southern Ocean low delta(13)C problem'. One explanation for this glacial distribution of delta(13)C calls upon surface productivity overprints or changes in the microhabitat of benthic foraminifera to lower glacial age delta(13)C values. We show here, however, that glacial-interglacial delta(13)C shifts are similarly large everywhere in the deep South Atlantic, regardless of productivity regime or sedimentary environment. Furthermore, the degree of isotopic decoupling between the Atlantic and Pacific basins is proportional to the magnitude of delta(13)C change in the Atlantic on all time scales. Thus, we conclude that the profoundly altered distribution of delta(13)C in the glacial Southern Ocean is most likely the result of deep ocean circulation changes. While the characteristics of the Southern Ocean delta(13)C records clearly point to reduced North Atlantic Deep Water input during glacial periods, the basinal differences suggest that the mode of Southern Ocean deep water formation must have been altered as well. (C) 2002 Elsevier Science B.V. All rights reserved.

2000
Channell, JET, Stoner JS, Hodell DA, Charles CD.  2000.  Geomagnetic paleointensity for the last 100 kyr from the sub-antarctic South Atlantic: a tool for inter-hemispheric correlation. Earth and Planetary Science Letters. 175:145-160.   10.1016/s0012-821x(99)00285-x   AbstractWebsite

We report relative paleointensity proxy records from four piston cores collected near the Agulhas Ridge and Meteor Rise (South Atlantic). The mean sedimentation rate of the cores varies from 24 cm/kyr to 11 cm/kyr. The two cores with mean sedimentation rates over 20 cm/kyr record positive remanence inclinations at 40-41 ka coeval with the Laschamp Event. Age models are based on oxygen isotope data from three of the cores, augmented by radiocarbon ages from nearby Core RC11-83, and by correlation of paleointensity records for the one core with no oxygen isotope data. The relative paleointensity proxy records are the first from the South Atlantic and from the high to mid-latitude southern hemisphere. Prominent paleointensity lows at similar to 40 ka and similar to 65 ka, as well as many other features, can be correlated to paleointensity records of comparable resolution from the northern hemisphere. The records are attributable, in large part, to the global-scale field, and therefore have potential for inter-hemispheric correlation at a resolution difficult to achieve with isotope data alone. (C) 2000 Elsevier Science B.V. All rights reserved.

1997
Charles, CD, Hunter DE, Fairbanks RG.  1997.  Interaction between the ENSO and the Asian monsoon in a coral record of tropical climate. Science. 277:925-928.   10.1126/science.277.5328.925   AbstractWebsite

The oxygen isotopic composition of a banded coral from the western equatorial Indian Ocean provides a 150-year-long history of the relation between the El Nino-Southern Oscillation (ENSO) phenomenon and the Asian monsoon. Interannual cycles in the coral time series were found to correlate with Pacific coral and instrumental climate records, suggesting a consistent linkage across ocean basins, despite the changing frequency and amplitude of the ENSO. However, decadal variability that is characteristic of the monsoon system also dominates the coral record, which implies important interactions between tropical and midlatitude climate variability. One prominent manifestation of this interaction is the strong amplitude modulation of the quasi-biennial cycle.