Publications

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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.

2010
Charles, CD, Pahnke K, Zahn R, Mortyn PG, Ninnemann U, Hodell DA.  2010.  Millennial scale evolution of the Southern Ocean chemical divide. Quaternary Science Reviews. 29:399-409.   10.1016/j.quascirev.2009.09.021   AbstractWebsite

The chemical properties of the mid-depth and deep Southern Ocean are diagnostic of the mechanisms of abrupt changes in the global ocean throughout the late Pleistocene, because the regional water mass conversion and mixing help determine global ocean gradients. Here we define continuous time series of Southern Ocean vertical gradients by differencing the records from two high deposition rate deep sea sedimentary sequences that span the last several ice age cycles. The inferred changes in vertical carbon and oxygen isotopic gradients were dominated by variability on the millennial scale, and they followed closely the abrupt climate events of the high latitude Northern Hemisphere. In particular, the stadial events of at least the last 200 kyr were characterized by enhanced mid-deep gradients in both delta(13)C (dissolved inorganic carbon) and delta(18)O (temperature). Interstadial events, conversely, featured reduced vertical gradients in both properties. The glacial terminations represented exceptions to this pattern of variability, as the vertical carbon isotopic gradient flattened dramatically at times of peak warmth in the Southern Ocean surface waters and with little or no corresponding change delta(18)O gradient. The available evidence suggests that properties of the upper layer of the Southern Ocean (Antarctic Intermediate Water) were influenced by an atmospherically mediated teleconnection to high latitude Northern Hemisphere. (C) 2009 Elsevier Ltd. All rights reserved.

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
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
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.

Schrag, DP, Adkins JF, McIntyre K, Alexander JL, Hodell DA, Charles CD, McManus JF.  2002.  The oxygen isotopic composition of seawater during the Last Glacial Maximum. Quaternary Science Reviews. 21:331-342.   10.1016/s0277-3791(01)00110-x   AbstractWebsite

High-resolution oxygen and hydrogen isotope measurements were made on pore fluids from deep-sea sediments from sites in the North and South Atlantic. The data provide direct measurements of changes in the isotopic composition of bottom waters during the Last Glacial Maximum (LGM). Results from Ocean Drilling Program (ODP) Site 981 in the North Atlantic, currently bathed in North Atlantic Deep Water (NADW) reproduces previous results from the Ceara and Bermuda Rises, constraining the glacial-interglacial change in delta(18)O of the deep Atlantic to be 0.7-0.8parts per thousand. Results from Site 984, which is located north of Site 981 and at a shallower water depth, yield a similar value (0.8parts per thousand), providing insight into the properties of Glacial North Atlantic Intermediate Water (GNAIW). Sites from ODP Leg 177 in the South Atlantic span the modern boundary between northern and southern sources of deep water. Data from the northern site (1088) yield a similar result to sites in the tropical and North Atlantic (0.7parts per thousand). At the southern site (1093), located south of the polar front, the change is substantially larger (1.1parts per thousand), representing the change in delta(18)O of southern source waters since the LGM. These results confirm previous estimates that the global average change in delta(18)O of seawater is 1.0+/-0.1parts per thousand. Hydrogen isotopes measured on pore fluids from three sites are consistent with the oxygen isotopes from these locations, giving further support to these results. At all sites studied, the temperature of the deep ocean during the LGM, calculated by combining the pore fluid results with oxygen isotope data from benthic foraminifera, was within VC of the freezing point of seawater. (C) 2001 Elsevier Science Ltd. 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.

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.

1997
Fairbanks, RG, Evans MN, Rubenstone JL, Mortlock RA, Broad K, Moore MD, Charles CD.  1997.  Evaluating climate indices and their geochemical proxies measured in corals. Coral Reefs. 16:S93-S100.   10.1007/s003380050245   AbstractWebsite

Standard ocean/climate indices such as the Nino-3 sea surface temperature (SST) index, based on sparse instrumental data, and atmospheric indices such as the Southern Oscillation Index (SOI), may now be substituted and/or extended by coral-based indices. Several elements or their isotopes are incorporated in coral aragonitic skeletons at predictable concentrations, some of which are temperature or salinity dependent. The availability of century-old corals, at key oceanographic sites, permits the establishment of a network of proxy climate indices.

Ninnemann, US, Charles CD.  1997.  Regional differences in Quaternary Subantarctic nutrient cycling: Link to intermediate and deep water ventilation. Paleoceanography. 12:560-567.   10.1029/97pa01032   AbstractWebsite

Several fundamental issues regarding carbon cycling in the glacial oceans rest on the development: of reliable descriptions of high southern latitude surface waters, Here we compare new Subantarctic planktonic foraminiferal delta(13)C records with previously published records to demonstrate two distinct regional patterns over glacial cycles: (1) a lour-amplitude signal (similar to 0.7 parts per thousand), previously observed in the Indian (primarily in Globigerina bulloides), that also dominates the Pacific, and (2) a higher-amplitude signal, previously observed in Neogloboquadrina Pachyderma, that is confined to the Atlantic and western Indian sectors. The near observations from the Southeast Pacific, a primary region of Antarctic Intermediate Water (AAIW) formation, strengthen the suggestion that intermediate water acted as a conduit for transferring delta(13)C variability to low latitudes, because the timing and amplitude of the Indo-Pacific low-amplitude delta(13)C changes are similar to those observed in planktonic records from the tropical Pacific and Atlantic. A new benthic foraminiferal delta(13)C record from intermediate depths in the South Atlantic is also similar to the Southeast Pacific surface water records, further demonstrating that this link between high- and low-latitude surface: waters might be maintained. The widespread Indo-Pacific Subantarctic surface water signal is obscured in records from the Atlantic sector by the large glacial-interglacial delta(13)C signal (>1.0 parts per thousand) that is most likely the result of nutrient changes related to variable North Atlantic Deep Water (NADW) production, pachyderma delta(13)C records is not matched in a new C, bulloides delta(13)C record. The confined regional extent of the high-amplitude signal and the discrepancy between the two species suggest that most of the excess nutrients in the glacial Atlantic (inferred from delta(13)C) may be removed seasonally by increased production in the Subantarctic.

1993
Shemesh, A, Macko SA, Charles CD, Rau GH.  1993.  Isotopic Evidence for Reduced Productivity in the Glacial Southern-Ocean. Science. 262:407-410.   10.1126/science.262.5132.407   AbstractWebsite

Records of carbon and nitrogen isotopes in biogenic silica and carbon isotopes in planktonic foraminifera from deep-sea sediment cores from the Southern Ocean reveal that the primary production during the last glacial maximum was lower than Holocene productivity. These observations conflict with the hypothesis that the low atmospheric carbon dioxide concentrations were introduced by an increase in the efficiency of the high-latitude biological pump. Instead, different oceanic sectors may have had high glacial productivity, or alternative mechanisms that do not involve the biological pump must be considered as the primary cause of the low glacial atmospheric carbon dioxide concentrations.

1992
Shemesh, A, Charles CD, Fairbanks RG.  1992.  Oxygen Isotopes in Biogenic Silica - Global Changes in Ocean Temperature and Isotopic Composition. Science. 256:1434-1436.   10.1126/science.256.5062.1434   AbstractWebsite

A record of oxygen isotopes in biogenic silica from a deep-sea sediment core from the Southern Ocean reveals that marine diatoms retain their primary isotopic composition after burial. As a result, the marine diatom record can be combined with data on coexisting planktonic foraminifera to monitor past surface temperature and isotopic composition of seawater. The coupling of these two records allows the solution of two paleotemperature equations for each core interval. Data from a South Atlantic core show that the average delta-O-18 during the glacial period at this site was higher by about 1.3 per mil than average Holocene values, and that average glacial-age temperatures were not significantly different from average Holocene values.

Charles, CD, Fairbanks RG.  1992.  Evidence From Southern-Ocean Sediments for the Effect of North-Atlantic Deep-Water Flux on Climate. Nature. 355:416-419.   10.1038/355416a0   AbstractWebsite

The Southern Ocean is perhaps the only region where fluctuations in the global influence of North Atlantic Deep Water (NADW) can be monitored unambiguously in single deep-sea cores. A carbon isotope record from benthic foraminifera in a Southern Ocean core reveals large and rapid changes in the flux of NADW during the last deglaciation, and an abrupt increase in the NADW production rate which immediately preceded large-scale melting of the Northern Hemisphere ice sheets. This sudden strengthening of the NADW thermohaline cell provides strong evidence for the importance of NADW in glacial-interglacial climate change.