Publications

Export 8 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]
C
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.

Naqvi, WA, Charles CD, Fairbanks RG.  1994.  Carbon and Oxygen Isotopic Records of Benthic Foraminifera From the Northeast Indian-Ocean - Implications on Glacial-Interglacial Atmospheric CO2 Changes. Earth and Planetary Science Letters. 121:99-110.   10.1016/0012-821x(94)90034-5   AbstractWebsite

We present here continuous records of delta(18)O and delta(13)C in benthic foraminifera, extending well into the last ice age, in two piston cores from the Andaman Sea (sill depth similar to 1.3 km) and the Bay of Bengal (3 km). These show that, contrary to the previous reports, the glacial to interglacial shift in delta(13)C,at mid-depths in the Northeast Indian Ocean was indistinguishable from the mean oceanic delta(13)C change, negating a more vigorous renewal of intermediate waters globally during the glacial time. The corresponding delta(13)C Shift in deep waters is estimated to be about 50% larger than that reported previously. Jointly with some recent data from the Pacific, our results indicate a modest glacial-Holocene shift in the intermediate to deep water chemical gradients in the Indo-Pacific as a whole, implying that it was perhaps not the dominant mechanism for the glacial-interglacial atmospheric CO2 variations. Also, in conflict with previous work, our measurements suggest significant cooling of both the intermediate and deep waters during the glacial time. The high-resolution records from the Andaman Sea help reconstruct paleoenvironmental changes at intermediate depths during the last deglaciation. Rapid increases in delta(13)C occurring in two stages during the early deglaciation appear to have been caused by the fluctuations in the North Atlantic Deep Water production. A negative excursion in delta(13)C during the mid-deglaciation is ascribed to enhanced nutrient regeneration at mid-depths associated with the greatly intensified summer monsoon around the Pleistocene-Holocene boundary.

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.

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.

Charles, CD, LynchStieglitz J, Ninnemann US, Fairbanks RG.  1996.  Climate connections between the hemisphere revealed by deep sea sediment core ice core correlations. Earth and Planetary Science Letters. 142:19-27.   10.1016/0012-821x(96)00083-0   AbstractWebsite

Correlation of Southern Ocean deep sea sediment core records with ice core records of polar climate delineates with unprecedented detail the relationship between high latitude climate and the ocean's thermohaline circulation over the last 80,000 years. Our observations suggest that, while North Atlantic Deep Water variability manifests itself clearly in Southern Ocean nutrient proxy records over periods as short as 500 yr, this deep water variability did not promote a direct link between climate variability in the high latitudes of the two hemispheres on millennial timescales. In particular, the proxy records indicate that, on average, northern hemisphere climate fluctuations lagged those of the southern hemisphere by 1500 yr.

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.

Hodell, DA, Charles CD, Ninnemann US.  2000.  Comparison of interglacial stages in the South Atlantic sector of the southern ocean for the past 450 kyr: implifications for Marine Isotope Stage (MIS) 11. Global and Planetary Change. 24:7-26.   10.1016/s0921-8181(99)00069-7   AbstractWebsite

Oxygen and carbon isotopic gradients in surface waters were reconstructed for the past 450 kyr by analysis of the planktic foraminifer Neogloboquadrina pachyderma in cores located at approximately 43 degrees, 47 degrees, and 54 degrees S across the Polar Frontal Zone in the South Atlantic sector of the Southern Ocean. Comparison of the oxygen isotopic records for peak interglacial conditions during the past 450 kyr reveals that Marine Isotope Stage (MIS) ii was not substantially warmer than other interglacials at high southern latitudes, although the period of warmth lasted longer. The carbonate and carbon isotope chemistry of surface and deep water represent the truly distinctive aspects of Stage 11 in the Southern Ocean. Peak carbonate production occurred at high southern latitudes during MIS 11, resulting in light-colored, high-carbonate sediments deposited throughout the Southern Ocean above the lysocline. Carbon isotopic values of benthic foraminifera in cores bathed by Circumpolar Deep Water (CPDW) were highest during MIS11, suggesting strong input of North Atlantic Deep Water (NADW) to the Southern Ocean. Planktic delta(13)C values at high southern latitudes were also highest during MIS Il, which may reflect upwelling of CPDW with a greater contribution of NADW, lower whole-ocean nutrient inventories, higher gas exchange rates, and/or lowered alkalinity of Antarctic surface waters (resulting from carbonate precipitation south of the Polar Front). (C) 2000 Elsevier Science B.V. All rights reserved.

Zaunbrecher, LK, Cobb KM, Beck JW, Charles CD, Druffel ERM, Fairbanks RG, Griffin S, Sayani HR.  2010.  Coral records of central tropical Pacific radiocarbon variability during the last millennium. Paleoceanography. 25   10.1029/2009pa001788   AbstractWebsite

The relationship between decadal to centennial changes in ocean circulation and climate is difficult to discern using the sparse and discontinuous instrumental record of climate and, as such, represents a large uncertainty in coupled ocean-atmosphere general circulation models. We present new modern and fossil coral radiocarbon (Delta(14)C) records from Palmyra (6 degrees N, 162 degrees W) and Christmas (2 degrees N, 157 degrees W) islands to constrain central tropical Pacific ocean circulation changes during the last millennium. Seasonally to annually resolved coral Delta(14)C measurements from the 10th, 12th-17th, and 20th centuries do not contain significant interannual to decadal-scale variations, despite large changes in coral delta(18)O on these timescales. A centennial-scale increase in coral radiocarbon from the Medieval Climate Anomaly (similar to 900-1200 AD) to the Little Ice Age (similar to 1500-1800) can be largely explained by changes in the atmospheric Delta(14)C, as determined with a box model of Palmyra mixed layer Delta(14)C. However, large 12th century depletions in Palmyra coral Delta(14)C may reflect as much as a 100% increase in upwelling rates and/or a significant decrease in the Delta(14)C of higher-latitude source waters reaching the equatorial Pacific during this time. SEM photos reveal evidence for minor dissolution and addition of secondary aragonite in the fossil corals, but our results suggest that coral Delta(14)C is only compromised after moderate to severe diagenesis for these relatively young fossil corals.