Publications

Export 14 results:
Sort by: Author Title Type [ Year  (Desc)]
2018
Gottschalk, J, Hodell DA, Skinner LC, Crowhurst SJ, Jaccard SL, Charles C.  2018.  Past carbonate preservation events in the deep southeast Atlantic Ocean (Cape Basin) and their implications for atlantic overturning dynamics and marine carbon cycling. Paleoceanography and Paleoclimatology. 33:643-663.   10.1029/2018pa003353   AbstractWebsite

Micropaleontological and geochemical analyses reveal distinct millennial-scale increases in carbonate preservation in the deep Southeast Atlantic (Cape Basin) during strong and prolonged Greenland interstadials that are superimposed on long-term (orbital-scale) changes in carbonate burial. These data suggest carbonate oversaturation of the deep Atlantic and a strengthened Atlantic Meridional Overturning Circulation (AMOC) during the most intense Greenland interstadials. However, proxy evidence from outside the Cape Basin indicates that AMOC changes also occurred during weaker and shorter Greenland interstadials. Here we revisit the link between AMOC dynamics and carbonate saturation in the deep Cape Basin over the last 400 kyr (sediment cores TN057-21,TN057-10, and Ocean Drilling Program Site 1089) by reconstructing centennial changes in carbonate preservation using millimeter-scale X-ray fluorescence (XRF) scanning data. We observe close agreement between variations in XRF Ca/Ti, sedimentary carbonate content, and foraminiferal shell fragmentation, reflecting a common control primarily through changing deep water carbonate saturation. We suggest that the high-frequency (suborbital) component of the XRF Ca/Ti records indicates the fast and recurrent redistribution of carbonate ions in the Atlantic basin via the AMOC during both long/strong and short/weak North Atlantic climate anomalies. In contrast, the low-frequency (orbital) XRF Ca/Ti component is interpreted to reflect slow adjustments through carbonate compensation and/or changes in the deep ocean respired carbon content. Our findings emphasize the recurrent influence of rapid AMOC variations on the marine carbonate system during past glacial periods, providing a mechanism for transferring the impacts of North Atlantic climate anomalies to the global carbon cycle via the Southern Ocean.

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
Lynch-Stieglitz, J, Curry WB, Oppo DW, Ninneman US, Charles CD, Munson J.  2006.  Meridional overturning circulation in the South Atlantic at the last glacial maximum. Geochemistry Geophysics Geosystems. 7   10.1029/2005gc001226   AbstractWebsite

The geostrophic shear associated with the meridional overturning circulation is reflected in the difference in density between the eastern and western margins of the ocean basin. Here we examine how the density difference across 30 degrees S in the upper 2 km of the Atlantic Ocean ( and thus the magnitude of the shear associated with the overturning circulation) has changed between the last glacial maximum and the present. We use oxygen isotope measurements on benthic foraminifera to reconstruct density. Today, the density in upper and intermediate waters along the eastern margin in the South Atlantic is greater than along the western margin, reflecting the vertical shear associated with the northward flow of surface and intermediate waters and the southward flowing North Atlantic Deep Waters below. The greater density along the eastern margin is reflected in the higher delta(18)O values for surface sediment benthic foraminifera than those found on the western margin for the upper 2 km. For the last glacial maximum the available data indicate that the eastern margin foraminifera had similar delta(18)O to those on the western margin between 1 and 2 km and that the gradient was reversed relative to today with the higher delta(18)O values in the western margin benthic foraminifera above 1 km. If this reversal in benthic foraminifera delta(18)O gradient reflects a reversal in seawater density gradient, these data are not consistent with a vigorous but shallower overturning cell in which surface waters entering the Atlantic basin are balanced by the southward export of Glacial North Atlantic Intermediate Water.

2003
Stoner, JS, Channell JET, Hodell DA, Charles CD.  2003.  A similar to 580 kyr paleomagnetic record from the sub-Antarctic South Atlantic (Ocean Drilling Program Site 1089). Journal of Geophysical Research-Solid Earth. 108   10.1029/2001jb001390   AbstractWebsite

We report geomagnetic directional paleosecular variation, relative paleointensity proxies and oxygen isotope data from the upper 88 m composite depth (mcd) at South Atlantic Ocean Drilling Program (ODP) Site 1089 (40degrees56.2'S, 9degrees53.64'E, 4620 m water depth). The age model is provided by high-resolution oxygen isotope stratigraphy, augmented by radiocarbon dates from the upper 8 mcd of nearby piston core RC11-83. Mean sedimentation rates at Site 1089 are in the range of 15 to 20 cm/kyr. Two intervals during the Brunhes Chron, at similar to29.6 mcd (similar to190 ka) and at similar to48 mcd (similar to335 ka), have component magnetization directions with positive (reverse polarity) inclination; however, the excursional directions are heavily overprinted by the postexcursional field. Magnetite is the dominant carrier of magnetic remanence, and occurs in the pseudosingle-domain (PSD) grain size. An additional higher-coercivity magnetic carrier, characterized by low unblocking temperatures (<350&DEG;C), is assumed to be authigenic pyrrhotite. A decrease in magnetization intensity down core is mirrored by a reduction in pore water sulfate, indicating diagenetic reduction of magnetite. Despite down-core changes in magnetic mineralogy, normalized intensity records from Site 1089 are comparable with high-resolution paleointensity records from the North Atlantic (e.g., ODP Sites 983 and 984). Sediment properties and sedimentation patterns within the Cape (Site 1089) and Iceland (Sites 983 and 984) Basins are distinctly different at both millennial and orbital timescales and therefore preclude lithologic variability from being the source of this correlation. Variations in normalized intensity from Site 1089 therefore appear to reflect changes in global-scale geomagnetic field intensity.

Hodell, DA, Venz KA, Charles CD, Ninnemann US.  2003.  Pleistocene vertical carbon isotope and carbonate gradients in the South Atlantic sector of the Southern Ocean. Geochemistry Geophysics Geosystems. 4   10.1029/2002gc000367   AbstractWebsite

[1] We demonstrate that the carbon isotopic signal of mid-depth waters evolved differently from deep waters in the South Atlantic sector of the Southern Ocean during the Pleistocene. Deep sites (>3700 m) exhibit large glacial-to-interglacial variations in benthic delta(13)C, whereas the amplitude of the delta(13)C signal at Site 1088 ( 2100 m water depth) is small. Unlike the deep sites, at no time during the Pleistocene were benthic delta(13)C values at Site 1088 lower than those of the deep Pacific. Reconstruction of intermediate-to-deep delta(13)C gradients (Delta(13)C(I-D)) supports the existence of a sharp chemocline between 2100 and 2700 m during most glacial stages of the last 1.1 myr. This chemical divide in the glacial Southern Ocean separated well-ventilated water above similar to2500 m from poorly ventilated water below. The Delta(13)C(I-D) signal parallels the Vostok atmospheric pCO(2) record for the last 400 kyr, lending support to physical models that invoke changes in Southern Ocean deep water ventilation as a mechanism for changing atmospheric pCO2. The emergence of a strong 100-kyr cycle in Delta(13)C(I-D) during the mid-Pleistocene supports a change in vertical fractionation and deep-water ventilation rates in the Southern Ocean, and is consistent with possible CO(2)-forcing of this climate transition.

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
Mortyn, P, Charles CD, Hodell DA.  2002.  Southern Ocean upper water column structure over the last 140 kyr with emphasis on the glacial terminations. Global and Planetary Change. 34:241-252.   10.1016/s0921-8181(02)00118-2   AbstractWebsite

We use delta(18)0 and delta(13)C isotopic records from both shallow- and deep-dwelling planktonic foraminiferal species to describe the transition of surface water column structure over the last two glacial terminations in the Atlantic sector of the Southern Ocean between 41degreesS-50degreesS. The delta(18)O differences between deep- and shallow-dwelling foraminifera are typically less pronounced before and during a termination than they are following a termination. Additionally, delta(13)C minima occur during terminations that may be linked to changes in thermohaline circulation mode. These observations collectively point toward water column structural changes that vary from relatively unstratified to relatively stratified during the course of a typical climate transition. The data can potentially be explained by southward retreats of surface water masses that previously advanced to the north during a typical glacial (i.e. frontal migration). Some of the delta(18)O phenomena can also be explained by subduction, entrainment, and advection of continental ice meltwater from higher latitudes. In either case, the glacial subantarctic between 41degreesS-50degreesS was less thermally stratified than it is today. (C) 2002 Elsevier Science B.V. All rights reserved.

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.

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
Hodell, DA, Kanfoush SL, Shemesh A, Crosta X, Charles CD, Guilderson TP.  2001.  Abrupt cooling of Antarctic surface waters and sea ice expansion in the South Atlantic sector of the Southern Ocean at 5000 cal yr B.P. Quaternary Research. 56:191-198.   10.1006/qres.2001.2252   AbstractWebsite

Antarctic surface waters were warm and ice free between 10,000 and 5000 cal yr B.P., as judged from ice-rafted debris and microfossils in a piston core at 53 degreesS in the South Atlantic. This evidence shows that about 5000 cal yr B.P., sea surface temperatures cooled, sea ice advanced, and the delivery of ice-rafted detritus (IRD) to the subantarctic South Atlantic increased abruptly. These changes mark the end of the Hypsithermal and onset of Neoglacial conditions. They coincide with an early Neoglacial advance of mountain glaciers in South America and New Zealand between 5400 and 4900 cal yr B.P., rapid middle Holocene climate changes inferred from the Taylor Dome Ice Core (Antarctica), cooling and increased IRD in the North Atlantic, and the end of the African humid period. The near synchrony and abruptness of all these climate changes suggest links among the tropics and both poles that involved nonlinear response to gradual changes in Northern Hemisphere insolation. Sea ice expansion in the Southern Ocean may have provided positive feedback that hastened the end of the Hypsithermal and African humid periods in the middle Holocene. (C) 2001 University of Washington.

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.

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.

1993
Charles, CD, Wright JD, Fairbanks RG.  1993.  Thermodynamic Influences on the Marine Carbon-Isotope Record. Paleoceanography. 8:691-697.   10.1029/93pa01803   AbstractWebsite

Air-sea exchange represents an important controlling factor for the distribution of carbon isotopes in the modern ocean, even below the surface mixed layer. Here we show that this thermodynamic influence varies between water masses with a magnitude which is large relative to the amplitude of typical deep sea sediment delta(13)C records. Thus, changes in air-sea exchange processes must be regarded as a significant source of variability for any marine delta(13)C signal.