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2019
Gruetzner, J, Espejo FJJ, Lathika N, Uenzelmann-Neben G, Hall IR, Hemming SR, LeVay LJ, Barker S, Berke MA, Brentegani L, Caley T, Cartagena-Sierra A, Charles CD, Coenen JJ, Crespin JG, Franzese AM, Han X, Hines SKV, Just J, Koutsodendris A, Kubota K, Norris RD, Santos TP, Robinson R, Rolinson JM, Simon MH, Tangunan D, van der Lubbe HJL, Yamane M, Zhang H, Expedition S.  2019.  A new seismic stratigraphy in the Indian-Atlantic Ocean gateway resembles major paleo-oceanographic changes of the last 7 Ma. Geochemistry Geophysics Geosystems. 20:339-358.   10.1029/2018gc007668   AbstractWebsite

The exchange of water masses between the Indian Ocean and the Atlantic constitutes an integral interocean link in the global thermohaline circulation. Long-term changes in deep water flow have been studied using seismic reflection profiles but the seismic stratigraphy was poorly constrained and not resolved for the time period from the late Miocene onward. Here we present results from International Ocean Discovery Program Site U1475 (Agulhas Plateau) located over a sediment drift proximal to the entrance of North Atlantic Deep Water into the Southern Ocean and South Indian Ocean. Site U1475 comprises a complete carbonate-rich stratigraphic section of the last similar to 7 Ma that provides an archive of climate-induced variations in ocean circulation. Six marker reflectors occurring in the upper 300 m of the drift are identified here for the first time. The formation of these reflectors is mainly due to density changes that are mostly caused by changes in biogenic versus terrigenous sediment deposition. Synthetic seismograms allow age assignments for the horizons based on biostratigraphy and magnetostratigraphy. Prominent reflectors are related to late Pleistocene glacial/interglacial variability, the middle and early Pleistocene transitions, and the onset of the northern hemisphere glaciation. A peculiar early Pliocene interval (similar to 5.3-4.0 Ma) bounded by two reflectors is characterized by fourfold elevated sedimentation rates (> 10 cm/kyr) and the occurrence of sediment waves. We argue that this enhanced sediment transport to the Agulhas Plateau was caused by a reorganization of the bottom current circulation pattern due to maximized inflow of North Atlantic Deep Water.

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.

2017
Linsley, BK, Wu HC, Rixen T, Charles CD, Gordon AL, Moore MD.  2017.  SPCZ zonal events and downstream influence on surface ocean conditions in the Indonesian Throughflow region. Geophysical Research Letters. 44:293-303.   10.1002/2016gl070985   AbstractWebsite

Seasonal surface freshening of the Makassar Strait, the main conduit of the Indonesian Throughflow (ITF), is a key factor controlling the ITF. Here we present a 262year reconstruction of seasonal sea-surface-salinity variability from 1742 to 2004 Common Era by using coral O-18 records from the central Makassar Strait. Our record reveals persistent seasonal freshening and also years with significant truncations of seasonal freshening that correlate exactly with South Pacific Convergence Zone (SPCZ) zonal events >4000km to the east. During these events, the SPCZ dramatically rotates similar to 15 degrees north to near the equator and stronger westward flowing South Pacific boundary currents force higher-salinity water through the Makassar Strait in February-May halting the normal seasonal freshening in the strait. By these teleconnections, our Makassar coral O-18 series provides the first record of the recurrence interval of these zonal SPCZ events and demonstrates that they have occurred on a semiregular basis since the mid-1700s.

2016
Sanchez, SC, Charles CD, Carriquiry JD, Villaescusa JA.  2016.  Two centuries of coherent decadal climate variability across the Pacific North American region. Geophysical Research Letters. 43:9208-9216.   10.1002/2016gl069037   AbstractWebsite

The decadal variability of the Pacific Ocean and North American hydroclimate are subjects of immediate concern for society, yet the length of the instrumental record limits full mechanistic understanding of this variability. Here we introduce a 178year, seasonally resolved coral oxygen isotopic record from Clarion Island (18 degrees N, 115 degrees W), a sampling a subtropical region that is strongly influenced by the decadal-scale fluctuations of the North Pacific Gyre Oscillation and a region that serves as a critical locus for the communication of climate anomalies with the tropics. This Mexican Pacific coral record is highly correlated to coral records from the central tropical Pacific and tree ring records from western North America. Significant changes in the amplitude of oceanic decadal variability in the early nineteenth century are mirrored in the drought reconstructions in western North America. The spatial manifestation of this relationship was relatively invariant, despite notable changes in the climatic mean state.

2014
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.

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.

Roach, LD, Charles CD, Field DB, Guilderson TP.  2013.  Foraminiferal radiocarbon record of northeast Pacific decadal subsurface variability. Journal of Geophysical Research-Oceans. 118:4317-4333.   10.1002/jgrc.20274   AbstractWebsite

The decadal dynamics of the subsurface North Pacific Ocean are largely inaccessible beyond sparse instrumental observations spanning the last 20 years. Here we present a approximate to 200 year long record of benthic foraminiferal radiocarbon (C-14), extracted at biennial resolution from the annually laminated sediments at the Santa Barbara Basin (SBB) depocenter (approximate to 600 m). The close match between core top benthic foraminiferal C-14 values and the C-14 of seawater dissolved inorganic carbon (DIC) suggests that benthic foraminifera faithfully capture the bottom water radiocarbon concentrations, as opposed to that of the deeper (>0.5 cm) sediment porewater zone. The full time series of benthic foraminiferal C-14 displays significant variability on decadal timescales, with excursions on the order of 40. These excursions are overprinted by a unidirectional trend over the late 20th century that likely reflects the sedimentary incorporation of bomb radiocarbon (via remineralized particulate organic carbon). We isolate this trend by means of a one-dimensional oxidation model, which considers the possible contribution of remineralized particles to the total ambient carbon pool. This oxidation model also considers the possible influence of carbon with a variety of sources (ages). Though variable oxidation of preaged carbon could exert a strong influence on benthic foraminiferal radiocarbon variability, the totality of evidence points to the vertical density structure along the Southern California Margin (SCM) as the primary driver of the SBB benthic foraminiferal C-14 record. For example, intervals characterized by significantly lower C-14 values correspond to periods of enhanced upwelling and subsurface equatorward flow along the SCM.

Carilli, JE, Charles CD, Garren M, McField M, Norris RD.  2013.  Baseline shifts in coral skeletal oxygen isotopic composition: a signature of symbiont shuffling? Coral Reefs. 32:559-571.   10.1007/s00338-012-1004-y   AbstractWebsite

Decades-long records of the stable isotopic composition of coral skeletal cores were analyzed from four sites on the Mesoamerican Reef. Two of the sites exhibited baseline shifts in oxygen isotopic composition after known coral bleaching events. Changes in pH at the calcification site caused by a change in the associated symbiont community are invoked to explain the observed shift in the isotopic composition. To test the hypothesis that changes in symbiont clade could affect skeletal chemistry, additional coral samples were collected from Belize for paired Symbiodinium identification and skeletal stable isotopic analysis. We found some evidence that skeletal stable isotopic composition may be affected by symbiont clade and suggest this is an important topic for future investigation. If different Symbiodinium clades leave consistent signatures in skeletal geochemical composition, the signature will provide a method to quantify past symbiont shuffling events, important for understanding how corals are likely to respond to climate change.

2012
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.

Rafter, PA, Sigman DM, Charles CD, Kaiser J, Haug GH.  2012.  Subsurface tropical Pacific nitrogen isotopic composition of nitrate: Biogeochemical signals and their transport. Global Biogeochemical Cycles. 26   10.1029/2010gb003979   AbstractWebsite

We report measurements of the nitrogen isotopic composition of nitrate (the delta(15)N of NO(3)(-)) across the equatorial Pacific, for zonal transects from 165 degrees E to 95 degrees W and meridional transects across 95 degrees and 110 degrees W. The delta(15)N of NO(3)(-) is similar in the equatorial thermocline (approximate to 100 m) and intermediate depth waters (approximate to 150 to 600 m), averaging (7.1 +/- 0.3)parts per thousand and (7.1 +/- 0.1)parts per thousand, respectively. These values are more than 2 parts per thousand higher than subthermocline waters of the Southern and Atlantic Oceans and are approximate to 1 parts per thousand higher than putative source waters in the high latitude South Pacific (Subantarctic Mode Water, SAMW). The combined constraints of nitrate concentration and delta(15)N of NO(3)(-) in the equatorial Pacific require (1) lateral exchange between the high-latitude source waters and the zones of denitrification in the eastern tropical Pacific and (2) the accumulation of remineralized nutrients at depth. The zonal uniformity of the subsurface equatorial Pacific delta(15)N of NO(3)(-) indicates rapid transport within the equatorial zone, which works to homogenize the delta(15)N of NO(3)(-) across the Pacific basin. Against this backdrop of high delta(15)N of NO(3)(-) in the tropical Pacific, we find a discrete off-equatorial core of lower delta(15)N of NO(3)(-) (5.5 +/- 0.3)parts per thousand concentrated at 5 degrees S and 150 to 200 m along the 110 degrees and 95 degrees W transects and in apparent association with the Southern Subsurface Counter Current (SSCC). We propose that the remineralized products of nitrogen fixation, at the source of the SSCC in the western south Pacific, are the origin of the low delta(15)N of NO(3)(-) in these waters.

2010
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.

Herguera, JC, Herbert T, Kashgarian M, Charles C.  2010.  Intermediate and deep water mass distribution in the Pacific during the Last Glacial Maximum inferred from oxygen and carbon stable isotopes. Quaternary Science Reviews. 29:1228-1245.   10.1016/j.quascirev.2010.02.009   AbstractWebsite

Intermediate ocean circulation changes during the last Glacial Maximum (LGM) in the North Pacific have been linked with Northern Hemisphere climate through air sea interactions, although the extent and the source of the variability of the processes forcing these changes are still not well resolved. The ventilated volumes and ages in the upper wind driven layer are related to the wind stress curl and surface buoyancy fluxes at mid to high latitudes in the North Pacific. In contrast, the deeper thermohaline layers are more effectively ventilated by direct atmosphere-sea exchange during convective formation of Subantarctic Mode Waters (SAMW) and Antarctic Intermediate Waters (AAIW) in the Southern Ocean, the precursors of Pacific Intermediate Waters (PIW) in the North Pacific. Results reported here show a fundamental change in the carbon isotopic gradient between intermediate and deep waters during the LGM in the eastern North Pacific indicating a deepening of nutrient and carbon rich waters. These observations suggest changes in the source and nature of intermediate waters of Southern Ocean origin that feed PIW and enhanced ventilation processes in the North Pacific, further affecting paleoproductivity and export patters in this basin. Furthermore, oxygen isotopic results indicate these changes may have been accomplished in part by changes in circulation affecting the intermediate depths during the LGM. (C) 2010 Elsevier Ltd. All rights reserved.

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.

Hartmann, AC, Carilli JE, Norris RD, Charles CD, Deheyn DD.  2010.  Stable isotopic records of bleaching and endolithic algae blooms in the skeleton of the boulder forming coral Montastraea faveolata. Coral Reefs. 29:1079-1089.   10.1007/s00338-010-0667-5   AbstractWebsite

Within boulder forming corals, fixation of dissolved inorganic carbon is performed by symbiotic dinoflagellates within the coral tissue and, to a lesser extent, endolithic algae within the coral skeleton. Endolithic algae produce distinctive green bands in the coral skeleton, and their origin may be related to periods of coral bleaching due to complete loss of dinoflagellate symbionts or "paling" in which symbiont populations are patchily reduced in coral tissue. Stable carbon isotopes were analyzed in coral skeletons across a known bleaching event and 12 blooms of endolithic algae to determine whether either of these types of changes in photosynthesis had a clear isotopic signature. Stable carbon isotopes tended to be enriched in the coral skeleton during the initiation of endolith blooms, consistent with enhanced photosynthesis by endoliths. In contrast, there were no consistent delta(13)C patterns directly associated with bleaching, suggesting that there is no unique isotopic signature of bleaching. On the other hand, isotopic values after bleaching were lighter 92% of the time when compared to the bleaching interval. This marked drop in skeletal delta(13)C may reflect increased kinetic fractionation and slow symbiont recolonization for several years after bleaching.

2009
Nurhati, IS, Cobb KM, Charles CD, Dunbar RB.  2009.  Late 20th century warming and freshening in the central tropical Pacific. Geophysical Research Letters. 36   10.1029/2009gl040270   AbstractWebsite

Global climate models and analyses of instrumental datasets provide a wide range of scenarios for future tropical Pacific climate change, limiting the accuracy of regional climate projections. Coral records provide continuous reconstructions of tropical Pacific climate trends that are difficult to quantify using the short, sparse instrumental datasets available from the tropical Pacific. Here, we present coral-based reconstructions of late 20th century sea-surface temperature and salinity trends from several islands in the central tropical Pacific. The coral data reveal warming trends that increase towards the equator, implying a decrease in equatorial upwelling in the last decades. Seawater freshening trends on the southern edge of the Inter-Tropical Convergence Zone suggest a strengthening and/or an equatorward shift of the convergence zone. Together, the new coral records support a late 20th century trend towards "El Nino-like" conditions in the tropical Pacific, in line with the majority of coupled global climate model projections. Citation: Nurhati, I. S., K. M. Cobb, C. D. Charles, and R. B. Dunbar (2009), Late 20th century warming and freshening in the central tropical Pacific, Geophys. Res. Lett., 36, L21606, doi:10.1029/2009GL040270.

Guilderson, TP, Fallon S, Moore MD, Schrag DP, Charles CD.  2009.  Seasonally resolved surface water Delta C-14 variability in the Lombok Strait: A coralline perspective. Journal of Geophysical Research-Oceans. 114   10.1029/2008jc004876   AbstractWebsite

We have explored surface water mixing in the Lombok Strait through a bimonthly resolved surface water Delta C-14 time series reconstructed from a coral in the Lombok Strait that spans 1937 through 1990. The prebomb surface water Delta C-14 average is -60.5 parts per thousand and individual samples range from -72 parts per thousand to 134 parts per thousand. The annual average postbomb maximum occurs in 1973 at 122 parts per thousand The timing of the postbomb maximum is consistent with a primary subtropical source for the surface waters in the Indonesian seas. During the postbomb period, the coral records regular seasonal cycles of 5 parts per thousand to 20 parts per thousand. Seasonal high Delta C-14 occur during March-May (warm, low salinity), and low Delta C-14 occur in September (cool, higher salinity). The Delta C-14 seasonality is coherent and in phase with the seasonal Delta C-14 cycle observed in Makassar Strait. We estimate the influence of high Delta C-14 Makassar Strait (North Pacific) water flowing through the Lombok Strait using a two end-member mixing model and the seasonal extremes observed at the two sites. The percentage of Makassar Strait water varies between 16 parts per thousand and 70 parts per thousand, and between 1955 and 1990, it averages at 40 parts per thousand. The rich Delta C-14 variability has a biennial component reflecting remote equatorial Indian Ocean forcing and a component in the ENSO band, which is interpreted to reflect Pacific forcing on the Delta C-14 signature in Lombok Strait.

2007
Lal, D, Charles C.  2007.  Deconvolution of the atmospheric radiocarbon record in the last 50,000 years. Earth and Planetary Science Letters. 258:550-560.   10.1016/j.epsl.2007.04.016   AbstractWebsite

Here we identify the principal causes of changes in the atmospheric Delta(14)C record by removing the influence of two discrete large changes in the geomagnetic field between 45 and 30 kyr. The marked transitions in the Delta(14)C record during this period can be interpreted as being the result of either "zero magnetic field" for periods of the order of 6500 y and 600 y at similar to 40 kyr and 32 kyr B.P, respectively, or for longer durations if the field was non-zero for appreciable time during the event. Once the effect of these events has been removed, the residual Delta(14)C record shows a plateau with nearly constant value at similar to 300 parts per thousand during 28-17 kyr B.P, followed by a sharp decrease in Delta(14)C between 17 kyr B.P. and the present, to a Delta(14)C value of similar to 0 parts per thousand. Estimated global paleomagnetic fields derived from paleointensity measurements in deep sea sediments (SINT-800, NAPIS-75 and Sumatra Basin) can only explain a maximum of 50% of this residual Delta(14)C record during 30-0 kyr B.P. We propose that the remainder must have resulted from changes in oceanic circulation leading to isolation of >= 20% of oceanic radiocarbon inventory from large scale mixing with the atmosphere during 28-17 kyr B.P. Subsequent inclusion of this carbon may be responsible in part for the decrease between 17 kyr B.P. to the present, to a Delta(14)C value of similar to 0 parts per thousand. We also note that the deconvoluted Delta(14)C record during the past 15,000 y B.P. seems to be primarily influenced by the changes in the atmospheric CO(2) concentrations. These possibilities have been discussed in previous works, but our results here point to a seemingly cyclical change in carbon exchange that was initiated prior to the Last Glacial Maximum. (c) 2007 Elsevier B.V. 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.

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.

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
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.

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.

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.

Cobb, KM, Charles CD, Cheng H, Kastner M, Edwards RL.  2003.  U/Th-dating living and young fossil corals from the central tropical Pacific. Earth and Planetary Science Letters. 210:91-103.   10.1016/s0012-821x(03)00138-9   AbstractWebsite

This study evaluates the accuracy of U/Th dates for young (< a few thousand years old) reef corals, both living and fossil, and explores strategies for refining those dates. The high precision of the U/Th method (+/-1-2%) for dating young corals is well-established. Earlier studies have demonstrated the method's accuracy for select samples of known age. However, these studies have focused on typical samples that have extremely low Th-232 concentrations (tens of pg/ g). Here we study the dating systematics of young corals that have low but significant amounts of Th-232 (up to 1000 pg/g), indicating the presence of small fractions of non-radiogenic Th-230 (i.e. Th-230 not generated by in situ U decay). We report U/Th ages for living and subaerially exposed fossil corals from Palmyra Island, located in the central tropical Pacific, that range from 50 to 700 yr old. The Palmyra corals contain varying amounts of Th-232 and small fractions of associated non-radiogenic Th-230. Uncertainty associated with the correction for non-radiogenic Th-230 can lead to significant errors in U/Th dates. We have characterized non-radiogenic Th-230/Th-232 values, (Th-230/Th-232)(nr), as a means of minimizing this source of error. We calculate (Th-230/Th-232)(nr), values ranging from 0 to 2 X 10(-5) for the Palmyra living corals by comparing measured U/Th dates to absolute dates for the living coral, whose chronology is well-established. For the fossil corals, we employ three different approaches to arrive at (Th-230/Th-232)(nr) estimates. First, we compare measured U/Th dates to absolute dates in samples from a young fossil coral that overlaps the living coral. Next, we use the firm relative dating constraints imposed by five overlapping fossil corals from the 14th-15th centuries to calculate (Th-230/Th-232)(nr) values. Finally, we attempt to anchor the 14th-15th century floating coral chronology to an absolute chronology by correlating the climate signals in the coral records to those in absolutely dated climate proxy records. All lines of evidence point to a range of (Th-230/Th-232)(nr) for fossil corals that overlaps the range determined for the living coral, suggesting that most of the thorium is primary or is added while the coral is still alive. Our work also demonstrates the utility of multiple (Th-230/Th-232)(nr) estimates. Most importantly, we demonstrate a method by which accurate (+/- 5 yr) U/Th-based chronologies can be obtained for young fossil corals with significant Th-232 concentrations.

Cobb, KM, Charles CD, Cheng H, Edwards RL.  2003.  El Nino/Southern Oscillation and tropical Pacific climate during the last millennium. Nature. 424:271-276.   10.1038/nature01779   AbstractWebsite

Any assessment of future climate change requires knowledge of the full range of natural variability in the El Nino/Southern Oscillation (ENSO) phenomenon. Here we splice together fossil-coral oxygen isotopic records from Palmyra Island in the tropical Pacific Ocean to provide 30-150-year windows of tropical Pacific climate variability within the last 1,100 years. The records indicate mean climate conditions in the central tropical Pacific ranging from relatively cool and dry during the tenth century to increasingly warmer and wetter climate in the twentieth century. But the corals also document a broad range of ENSO behaviour that correlates poorly with these estimates of mean climate. The most intense ENSO activity within the reconstruction occurred during the mid-seventeenth century. Taken together, the coral data imply that the majority of ENSO variability over the last millennium may have arisen from dynamics internal to the ENSO system itself.