Export 5 results:
Sort by: Author [ Title  (Asc)] Type Year
A B C D E F G H I J K L [M] N O P Q R S T U V W X Y Z   [Show ALL]
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.

Hodell, DA, Venz KA, Charles CD, Sierro FJ.  2002.  The mid-Brunhes transition in ODP sites 1089 and 1090. Earth's climate and orbital eccentricity : the marine isotope stage 11 question. 137( Droxler AW, Poore RZ, Burckle LH, Eds.)., Washington, DC: American Geophysical Union Abstract
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.

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.

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.