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.

2013
Cobb, KM, Westphal N, Sayani HR, Watson JT, Di Lorenzo E, Cheng H, Edwards RL, Charles CD.  2013.  Highly variable El Nino-Southern Oscillation throughout the Holocene. Science. 339:67-70.   10.1126/science.1228246   AbstractWebsite

The El Nino-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.

2012
Rafter, PA, Charles CD.  2012.  Pleistocene equatorial Pacific dynamics inferred from the zonal asymmetry in sedimentary nitrogen isotopes. Paleoceanography. 27   10.1029/2012pa002367   AbstractWebsite

There is little agreement among models in predicting the average state of the tropical Pacific when subjected to enhanced greenhouse gas forcing. This uncertainty emphasizes the importance of reconstructing past variability in tropical Pacific climate through episodes of significant and estimable radiative forcing. Thus far, efforts along these lines have concentrated primarily on inferences of sea surface temperature variability from deep-sea sediments. Here we offer a different view of the equatorial Pacific over the past similar to 1.2 million years (Myr)-before and after the mid-Pleistocene shift in the structure of ice ages. The zonal gradient in the nitrogen isotopic composition of sediment across the equatorial Pacific reflects nutrient delivery to the surface and, by extension, ocean dynamical properties. Over the last similar to 1.2 Myr, the variability of eastern equatorial Pacific nutrient upwelling (inferred from relative nitrate utilization) was highly correlated with local seasonal insolation. By contrast, nitrate utilization was insensitive to the 100,000 year cycle that dominated many other aspects of the Pleistocene ice ages, including greenhouse gas concentrations. A strong linear relationship between relative nitrate utilization and seasonal insolation over the past similar to 1 million years suggests a predictable response of one primary determinant of tropical Pacific climate change.

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.

2003
Cobb, KM, Charles CD, Cheng H, Edwards RL.  2003.  El Nino/Southern Oscillation and tropical Pacific climate during the last millennium. Nature. 424:271-276.   10.1038/nature01779   AbstractWebsite

Any assessment of future climate change requires knowledge of the full range of natural variability in the El Nino/Southern Oscillation (ENSO) phenomenon. Here we splice together fossil-coral oxygen isotopic records from Palmyra Island in the tropical Pacific Ocean to provide 30-150-year windows of tropical Pacific climate variability within the last 1,100 years. The records indicate mean climate conditions in the central tropical Pacific ranging from relatively cool and dry during the tenth century to increasingly warmer and wetter climate in the twentieth century. But the corals also document a broad range of ENSO behaviour that correlates poorly with these estimates of mean climate. The most intense ENSO activity within the reconstruction occurred during the mid-seventeenth century. Taken together, the coral data imply that the majority of ENSO variability over the last millennium may have arisen from dynamics internal to the ENSO system itself.

Flores, JA, Marino M, Sierro FJ, Hodell DA, Charles CD.  2003.  Calcareous plankton dissolution pattern and coccolithophore assemblages during the last 600 kyr at ODP Site 1089 (Cape Basin, South Atlantic): paleoceanographic implications. Palaeogeography Palaeoclimatology Palaeoecology. 196:409-426.   10.1016/s0031-0182(03)00467-x   AbstractWebsite

Coccolithophore assemblages at ODP Site 1089 in the southern Cape Basin (similar to41degreesS) were used to reconstruct surface-water conditions for the late Quaternary (Marine Isotope Stages (MIS) 1-15) in a region of strong hydrographic gradients in the southeast Atlantic. Stratigraphic control was provided by oxygen isotope stratigraphy and calcareous nannofossil events that are thought to be synchronous over a broad range of latitudes. The greatest coccolith abundances occurred at glacial terminations and, to a lesser degree, during glacial stages. Conversely, coccolithophores were the least abundant during the transition between interglacial to glacial stages, when calcium carbonate dissolution was strong. With the exception of these intervals, coccolith preservation is moderate to good, allowing study of the assemblages. The total abundance of coccolithophores and calcium carbonate variations at Site 1089 result both from variations in dissolution and carbonate production. During terminations, for example, the greatest calcium carbonate concentrations occurred at the same time as a moderate-to-poor preservation of coccoliths and foraminifers. Carbonate production was relatively high during these intervals. However, during terminations IV and V. maxima in carbonate production in the ocean were linked to high-dissolution processes at Site 1089. This trend is not observed for terminations I, II and III [Hodell et al., Earth Planet. Sci. Lett. 192 (2001) 109-124]. The interval from MIS 9 to 13 is coincident with high abundances of highly calcified species such as Gephyrocapsa caribbeanica. Here we discuss the contribution of this ubiquitous species to the production of calcium carbonate and their paleoecological significance. Except for occasional coccolith-barren intervals during interglacial periods, subtropical coccolith species were present continuously at ODP Site 1089 during the late Pleistocene. This suggests that the Polar Front has been south of Site 1089 for the last 600 kyr. (C) 2003 Elsevier Science B.V. All rights reserved.

Mortyn, PG, Charles CD, Ninnemann US, Ludwig K, Hodell DA.  2003.  Deep sea sedimentary analogs for the Vostok ice core. Geochemistry Geophysics Geosystems. 4   10.1029/2002gc000475   AbstractWebsite

Many applications of the Vostok ice core depend critically on the ability to make stratigraphic ties to marine records in the adjacent Southern Ocean. Here we present oxygen isotopic records from high accumulation rate sites in the South Atlantic sector of the Southern Ocean, collected for the purpose of complementing the recently extended deltaD record from the Vostok ice core. The combination of several planktonic foraminiferal delta(18)O records from northern subantarctic piston cores demonstrates that all of the millennial-scale oscillations expressed in the Vostok ice core over the last 60 ky are also present in marine records. The observations also support the assumption that the millennial-scale oscillations common to both marine and ice archives are synchronous, thus providing a rationale for extending the marine-ice core comparison through the last 400,000 years, making use of a marine drilled core (ODP Site 1089). By aligning the phase of these common abrupt events, we anchor the Vostok chronology to an orbitally tuned marine sediment chronology-a refinement that allows examination of a variety of paleoclimatological issues such as the relationship between deep ocean variability and Antarctic polar climate. For example, this exercise suggests that, over at least the 4 major deglaciation events, the primary (orbital scale) changes in the chemistry and, most likely, the temperature of the deep Southern ocean were synchronous with changes in atmospheric pCO(2) and polar air temperatures. We also find that the deuterium excess in the ice core resembles marine (foraminiferal) delta(13)C records and that the deuterium excess is synchronous with an "anomalous'' foraminiferal delta(18)O signal ( the residual between normalized versions of Vostok deltaD and foraminiferal delta(18)O). These observations demand a tight link between the Vostok isotopic record and the air-sea interaction of the subantarctic zone.

2002
Shemesh, A, Hodell D, Crosta X, Kanfoush S, Charles C, Guilderson T.  2002.  Sequence of events during the last deglaciation in Southern Ocean sediments and Antarctic ice cores. Paleoceanography. 17   10.1029/2000pa000599   AbstractWebsite

[1] The last glacial to interglacial transition was studied using down core records of stable isotopes in diatoms and foraminifera as well as surface water temperature, sea ice extent, and ice-rafted debris (IRD) concentrations from a piston core retrieved from the Atlantic sector of the Southern Ocean. Sea ice is the first variable to change during the last deglaciation, followed by nutrient proxies and sea surface temperature. This sequence of events is independent of the age model adopted for the core. The comparison of the marine records to Antarctic ice CO2 variation depends on the age model as C-14 determinations cannot be obtained for the time interval of 29.5-14.5 ka. Assuming a constant sedimentation rate for this interval, our data suggest that sea ice and nutrient changes at about 19 ka B. P. lead the increase in atmospheric pCO(2) by approximately 2000 years. Our diatom-based sea ice record is in phase with the sodium record of the Vostok ice core, which is related to sea ice cover and similarly leads the increase in atmospheric CO2. If gas exchange played a major role in determining glacial to interglacial CO2 variations, then a delay mechanism of a few thousand years is needed to explain the observed sequence of events. Otherwise, the main cause of atmospheric pCO(2) change must be sought elsewhere, rather than in the Southern Ocean.

Perks, HM, Charles CD, Keeling RF.  2002.  Precessionally forced productivity variations across the equatorial Pacific. Paleoceanography. 17   10.1029/2000pa000603   AbstractWebsite

[1] Measurements of combustion oxygen demand (COD) in two sediment cores provide a record of paleoproductivity driven by surface-ocean dynamics in the equatorial eastern and western Pacific for the past 400,000 years. The COD time series are well correlated with each other over this time span and show pronounced precessionally forced peaks of higher productivity during globally colder periods. The phase of this signal in the two cores is identical, to within chronological uncertainties, suggesting a common insolation forcing mechanism for the upper ocean across the equatorial Pacific. COD is also in phase with the precessionally forced component of global ice volume, as indicated by oxygen isotopes, and with atmospheric methane in the Vostok ice core. These relationships imply that the COD relative paleoproductivity index provides an important diagnostic measure of the mechanisms of tropical ocean dynamics and climate change.

Kanfoush, SL, Hodell DA, Charles CD, Janecek TR, Rack FR.  2002.  Comparison of ice-rafted debris and physical properties in ODP Site 1094 (South Atlantic) with the Vostok ice core over the last four climatic cycles. Palaeogeography Palaeoclimatology Palaeoecology. 182:329-349.   10.1016/s0031-0182(01)00502-8   AbstractWebsite

Visual counts of ice-grafted debris (IRD), foraminifera, and radiolaria were made for similar to1500 samples in Site 1094 spanning the last four climatic cycles (marine isotope stages 1-11). Most, but not all, of the IRD variability is captured by whole-core physical properties including magnetic susceptibility and gamma-ray attenuation bulk density. Glacial periods are marked by high IRD abundance and millennial-scale variability, which may reflect instability of ice shelves in the Weddell Sea region. Each interglacial period exhibits low IRD and high foraminiferal abundance during the early part of the interglacial, indicating relatively warm sea-surface temperatures and reduced influence of sea ice. IRD increases and foraminiferal abundances decrease during the latter part of each interglacial, indicating a return to more glacial-like conditions. Glacial terminations I and V are each characterized by a step-wise reduction in ice-rafting punctuated by a brief pulse in IRD delivery and reversal in delta(18)O. The coarse fraction of the sediment is dominated by ash and radiolaria, and the relative abundance of these components is remarkably similar to the concentration of Na+ in Vostok. Each of these variables is believed to be controlled mainly by sea-ice cover, thereby providing a means for sediment-ice core correlation. (C) 2002 Elsevier Science B.V. All rights reserved.

2001
Cobb, KM, Charles CD, Hunter DE.  2001.  A central tropical Pacific coral demonstrates Pacific, Indian, and Atlantic decadal climate connections. Geophysical Research Letters. 28:2209-2212.   10.1029/2001gl012919   AbstractWebsite

While instrumental and proxy-based climate records describe significant decadal-scale climate variability throughout the tropical Pacific, Indian, and Atlantic Oceans, the processes responsible for these variations and their interactions are not readily apparent from the observations. A new 112-yr coral-based sea surface temperature (SST) reconstruction from Palmyra Island in the central tropical Pacific (CTP) exhibits strong decadal variability with an amplitude of roughly 0.3 degreesC. A 12-13yr-period signal in this coral record is highly coherent with long equatorial Atlantic and Indian Ocean climate records, implying a unified phenomenon. The Atlantic pattern suggests that it may fall under direct influence of anomalous SST in the CTP, as it does over interannual timescales, while the Indian Ocean pattern exhibits maximum response during the switch between warm/cold states in the tropical Pacific. The results demonstrate that the CTP has played a significant role in determining the expression of global decadal climate variability over the twentieth century.

Charles, CD, Rind D, Healy R, Webb R.  2001.  Tropical cooling and the isotopic composition of precipitation in general circulation model simulations of the ice age climate. Climate Dynamics. 17:489-502.   10.1007/s003820000126   AbstractWebsite

We test the climate effects of changes in the tropical ocean by imposing three different patterns of tropical SSTs in ice age general circulation model simulations that include water source tracers and water isotope tracers. The continental air temperature and hydrological cycle response in these simulations is substantial and should be directly comparable to the paleoclimatic record. With tropical cooling imposed, there is a strong temperature response in mid- to high-latitudes resulting from changes in sea ice and disturbance of the planetary waves; the results suggest that tropical/subtropical ocean cooling leads to significant dynamical and radiative feedbacks that might amplify ice age cycles, The isotopes in precipitation generally follow the temperature response at higher latitudes, but regional delta O-18/air temperature scaling factors differ greatly among the experiments. In low-latitudes, continental surface temperatures decrease congruently with the adjacent SSTs in the cooling experiments. Assuming CLIMAP SSTs, O-18/O-16 ratios in low-latitude precipitation show no change from modern values. However, the experiments with additional cooling of SSTs produce much lower tropical continental delta O-18 values, and these low values result primarily from an enhanced recycling of continental moisture (as marine evaporation is reduced). The water isotopes are especially sensitive to continental aridity, suggesting that they represent an effective tracer of the extent of tropical cooling and drying. Only one of the tropical cooling simulations produces generalized low-latitude aridity. These results demonstrate that the geographic pattern of cooling is most critical for promoting much drier continents, and they underscore the need for accurate reconstructions of SST gradients in the ice age ocean.

2000
Kanfoush, SL, Hodell DA, Charles CD, Guilderson TP, Mortyn PG, Ninnemann US.  2000.  Millennial-scale instability of the antarctic ice sheet during the last glaciation. Science. 288:1815-1818.   10.1126/science.288.5472.1815   AbstractWebsite

Records of ice-rafted detritus (IRD) concentration in deep-sea cores from the southeast Atlantic Ocean reveal millennial-scale pulses of IRD delivery between 20,000 and 74,000 years ago. Prominent IRD Layers correlate across the Polar Frontal Zone, suggesting episodes of Antarctic Ice Sheet instability. Carbon isotopes (delta(13)C) of benthic foraminifers, a proxy of deepwater circulation, reveal that South Atlantic IRD events coincided with strong increases in North Atlantic Deep Water (NADW) production and inferred warming (interstadials) in the high-latitude North Atlantic. Sea level rise or increased NADW production associated with strong interstadials may have resulted in destabilization of grounded ice shelves and possible surging in the Weddell Sea region of West Antarctica.

Moore, MD, Charles CD, Rubenstone JL, Fairbanks RG.  2000.  U/Th-dated sclerosponges from the Indonesian Seaway record subsurface adjustments to west Pacific winds. Paleoceanography. 15:404-416.   10.1029/1999pa000396   AbstractWebsite

Stable isotope records from sclerosponges collected at 10-20 m depth in the Indonesian Seaway and Solomon Islands are particularly well suited for reconstructing century-scale trends in ambient temperature variability and the oceanic uptake of fossil fuel carbon. Basal U/Th dates demonstrate that the sclerosponges analyzed are 85-100 years old. Isotopic records from the Indonesian specimens suggest a strong subsurface cooling over the past 20 years that is not manifested in either surface instrumental or shallower coral proxy records. However, analysis of observed subsurface temperatures in Indonesia, observed winds in the west Pacific, and simulated subsurface temperatures from a steady state general circulation model hindcast forced by observed winds combine to suggest that thermocline adjustments could account for at least part of the recent cooling inferred from the Indonesian sclerosponges. If so, the sclerosponge data suggests that, on average, the west Pacific thermocline has shoaled significantly over at least the past 2 decades.