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Charles, C.  1998.  Palaeoclimatology - The ends of an era. Nature. 394:422-423.   10.1038/28741   AbstractWebsite
Lal, D, Charles C, Vacher L, Goswami JN, Jull AJT, McHargue L, Finkel RC.  2006.  Paleo-ocean chemistry records in marine opal: Implications for fluxes of trace elements, cosmogenic nuclides (Be-10 and Al-26), and biological productivity. Geochimica Et Cosmochimica Acta. 70:3275-3289.   10.1016/j.gca.2006.04.004   AbstractWebsite

Here, we provide evidence suggesting that marine (diatom) opal contains not only a high fidelity record of dissolved oceanic concentrations of cosmic ray-produced radionuclides, Be-10 and Al-26, but also a record of temporal variations in a large number of trace elements such as Ti, Fe, Zn and Mn. This finding is derived from measurements in purified biogenic opal that can be separated from detrital materials using a newly developed technique based on surface charge characteristics. Initial results from a sediment core taken near the present-day position of the Antarctic Polar Front (ODP Site 1093) show dramatic changes in the intrinsic concentrations of, Be, Al, Ti, Fe, Mn and Zn in the opal assemblages during the past similar to 140 kyr BP. The results imply appreciable climatically controlled fluctuations in the level of bioreactive trace elements. The time series of total Be. Al, Ti, Fe and 10Be in the sediment core are all well correlated with each other and with dust records in the polar ice cores. The observations suggest that a significant flux of these trace metals to oceans is contributed by the aeolian dust, in this case, presumably from the Patagonia. This observation also allows determination of fluxes of dust-contributed Be-10 to the Antarctica ice sheets. However. our data show that the relationships among the various metals are not perfectly linear. During periods of higher dissolved concentrations of trace elements (indicated by Fe and Ti) the relative concentrations of bioreactive elements, Be, Al, Mn and Zn are decreased. By contrast. the Fe/Zn and Fe/Mn ratios decrease significantly during each transition from cold to warm periods. The relative behavior could be consistent with any of the following processes: (i) enhanced biological productivity due to greater supply of the bioreactive elements (e.g. Zn) during cold periods (ii) increased biological and inorganic scavenging of particle active elements (e.g. Be and Al) during early interglacial periods (iii) differential uptake/removal of the metals by the various diatom taxa whose relative productivity or growth rate changes with large scale climate. In any case. with one sedimentary phase and in single sedimentary sections, we now have the potential to compare directly a proxy for aeolian input of micronutrients (e.g. Fe or Ti), with a proxy for production (e.g. Al-26/Al ratios). We expect that studies of the temporal records of trace elements and cosmogenic nuclides in contrasting regions of upwelling and productivity, which exhibit different sensitivities to global climate fluctuations and micronutrient inputs, would lead to a direct and comprehensive test of ideas such as the hypothesis of iron control of atmospheric carbon dioxide [Martin, J.H., 1990. Glacial-interglacial CO2 change: the iron hypothesis. Paleoceanography 5, 1-13]. Our present data from a single site do not show that increases in dissolved Fe concentrations, per se, were responsible for increased biological productivity. However, a much clearer picture of the effect of increased dust fluxes should emerge when we have data for trace elements and the cosmogenic nuclides, 10Be and 26 Al from various oceanic provinces. (c) 2006 Elsevier Inc. All rights reserved.

Charles, CD, Morley JJ.  1988.  The Paleoceanographic Significance of the Radiolarian Didymocyrtis-Tetrathalamus in Eastern Cape Basin Sediments. Palaeogeography Palaeoclimatology Palaeoecology. 66:113-126.   10.1016/0031-0182(88)90084-3   AbstractWebsite
Van De Geer, G, Heusser LE, Lynch-Stieglitz J, Charles CD.  1994.  Paleoenvironments of Tasmania inferred from a 5-75 KA marine pollen record. Palynology. 18:33-40. AbstractWebsite

Pollen and oxygen isotope analyses of sediments from deep-sea core SO36-7SL (42 degree 18' S, 144 degree 40' E; 1085-m water depth) provide a chronostratigraphically-controlled paleoenvironmental record of the last 75,000 years from southeastern Australia. During the early Holocene and later part of the preceding period of deglaciation (post-14,000 yr B.P.), the vegetation of coastal central western Tasmania consisted mainly of Phyllocladus Eucalyptus mixed forest with a mesic understory, implying warm and humid climate. Grassy herbland communities with scattered Athrotaxis and Lagarostrobos were widespread during the last major phase of highland glaciation ( apprx 25,000-14,000 yr B.P.), and reflect much lower temperatures than at present. During much of the last glaciation ( apprx 63,000-25,000 yr B.P.), the vegetation was a mosaic of Eucalyptus woodland, shrubland, herbland and sedgeland communities that developed under generally cool and moist climate. Very high Asteraceae and low Eucalyptus, Phyllocladus and Dicksonia/Cyathea pollen and spore values indicate colder and probably drier environments apprx 70,000-63,000 yr B.P. The basal Eucalyptus-Dicksonia/Cyathea-Phyllocladius forest assemblage, deposited -75,000-70,000 yr B.P., signifies a rapid transition from a moderately warm and moist interglacial-like climate to the colder conditions and open environments that characterized much of the last glacial period. The results of the past 50,000 years are broadly consistent with paleoenvironmental reconstructions inferred from radiocarbon dated lowland pollen sequences in western Tasmania and further afield.

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.

Turner, SK, Sexton PF, Charles CD, Norris RD.  2014.  Persistence of carbon release events through the peak of early Eocene global warmth. Nature Geoscience. 7:748-751.   10.1038/ngeo2240   AbstractWebsite

The Early Eocene Climatic Optimum (53-50 million years ago) was preceded by approximately six million years of progressive global warming(1). This warming was punctuated by a series of rapid hyperthermal warming events triggered by the release of greenhouse gases(1-7). Over these six million years, the carbon isotope record suggests that the events became more frequent but smaller in magnitude(3,5-7). This pattern has been suggested to reflect a thermodynamic threshold for carbon release that was more easily crossed as global temperature rose, combined with a decrease in the size of carbon reservoirs during extremely warm conditions(8-11). Here we present a continuous, 4.25-million-year-long record of the stable isotope composition of carbonate sediments from the equatorial Atlantic, spanning the peak of early Eocene global warmth. A composite of this and pre-existing(7,12) records shows that the carbon isotope excursions that identify the hyperthermals exhibit continuity in magnitude and frequency throughout the approximately 10-million-year period covering the onset, peak and termination of the Early Eocene Climate Optimum. We suggest that the carbon cycle processes behind these events, excluding the largest event, the Palaeocene-Eocene Thermal Maximum (about 56 million years ago), were not exceptional. Instead, we argue that the hyperthermals may reflect orbital forcing of the carbon cycle analogous to the mechanisms proposed(13,14) to operate in the cooler Oligocene and Miocene.

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.

Mortyn, PG, Charles CD.  2003.  Planktonic foraminiferal depth habitat and delta O-18 calibrations: Plankton tow results from the Atlantic sector of the Southern Ocean. Paleoceanography. 18   10.1029/2001pa000637   AbstractWebsite

[1] Plankton tows conducted in the Atlantic sector of the Southern Ocean allow analysis of the influence of water column structure on planktonic foraminiferal abundance and delta(18)O composition. Foraminiferal abundance varies by several orders of magnitude across a large gradient in sea surface temperature and other hydrographic features, demonstrating high sensitivity of foraminiferal populations to regional differences in water properties. The depth of maximum abundance for key species such as Globigerina bulloides and Neogloboquadrina pachyderma is not constant from station to station. The pattern suggests that their abundance and shell chemistry are tied to density horizons or other conditions (such as food availability) that become more sharply defined with depth in the northern subantarctic. The consistent observation of Globorotalia inflata and Globoratalia truncatulinoides as relatively deep-dwelling species confirms their utility as indicators of upper thermocline properties. In delta(18)O all species are observed to be isotopically lighter than predicted from water properties, but the species-specific offset is fairly uniform at all stations. These observations define the utility of multispecies delta(18)O for reconstructing temperature and density stratification from past surface oceans.

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.

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.

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.