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Anderson, PA, Charles CD, Berger WH.  2001.  Walvis Paradox confirmed for the early Quaternary at the southern end of the Namibia upwelling system, ODP Site 1085. Proceedings of the Ocean Drilling Program Scientific Results. 175   10.2973/odp.proc.sr.175.201.2001   Abstract

The Walvis Paradox states that opal accumulation fluctuates in counterphase to general productivity on Walvis Ridge, off northern Namibia. Sediments of early Quaternary age from Site 1085 (Cores 175-1085A-7H to 10H, off South Africa) were studied to check the phase relationships of general productivity and opal deposition in the region off the Oranje River, near the southern end of the Namibia upwelling system. The proxies used are 18O of Cibicidoides wuellerstorfi, 13C of C. wuellerstorfi, benthic foraminifers per gram (BF/g), Uvigerina spp. per gram (U/g), estimated diatom abundance (EDA) and sulfide and oxide aggregates (SOA) in the coarse fraction. EDA and BF/g are taken to indicate diatom and organic matter productivity, respectively. Oxygen isotopes were used to determine phase within the glacial-interglacial cycles.The phase relationships between the different proxies emerge when applying internal stacking methods. For Core 175-1085A-7H, five prescribed 41-k.y. cycles were combined into an average 41-k.y. cycle for the different indices. Productivity indices (BF/g and U/g) tend to follow 18O in the accustomed manner (with glacial periods showing maximum productivity), but not EDA, whose maximum appears closer to interglacials, offset toward cooling. Thus, phase relationships are similar to those on Walvis Ridge, extending the Walvis Paradox to the southern end of the Namibia upwelling system for the early Quaternary. A tentative reconstruction of the phase of intensity of mixing is given, based on the two productivity indicators EDA and BF/g. Mixing is strongest during the glacial maximum.

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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.

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.

Charles, C.  1998.  Palaeoclimatology - The ends of an era. Nature. 394:422-423.   10.1038/28741   AbstractWebsite
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Charles, CD, Fairbanks RG.  1992.  Evidence From Southern-Ocean Sediments for the Effect of North-Atlantic Deep-Water Flux on Climate. Nature. 355:416-419.   10.1038/355416a0   AbstractWebsite

The Southern Ocean is perhaps the only region where fluctuations in the global influence of North Atlantic Deep Water (NADW) can be monitored unambiguously in single deep-sea cores. A carbon isotope record from benthic foraminifera in a Southern Ocean core reveals large and rapid changes in the flux of NADW during the last deglaciation, and an abrupt increase in the NADW production rate which immediately preceded large-scale melting of the Northern Hemisphere ice sheets. This sudden strengthening of the NADW thermohaline cell provides strong evidence for the importance of NADW in glacial-interglacial climate change.

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
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Charles, CD, Rind D, Jouzel J, Koster RD, Fairbanks RG.  1995.  Seasonal Precipitation Timing and Ice Core Records. Science. 269:247-248.   10.1126/science.269.5221.247   AbstractWebsite
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Charles, CD, Froelich PN, Zibello MA, Mortlock RA, Morley JJ.  1991.  Biogenic Opal in Southern Ocean Sediments Over the Last 450,000 Years: Implications for Surface Water Chemistry and Circulation. Paleoceanography. 6:697-728.   10.1029/91pa02477   AbstractWebsite

We present records of biogenic opal percentage and burial rate in 12 piston cores from the Atlantic and Indian sectors of the Southern Ocean. These records provide a detailed, quantitative description of changing patterns of opal deposition over the last 450 kyr. The striking regional coherence of these records suggests that dissolution in the deep sea and sediment pore waters does not obscure the surface productivity signal, and therefore these opal time series can be used in combination with other surface water tracers to make inferences about the chemistry and circulation of the Southern Ocean under different global climate conditions. Three broad depositional patterns can be distinguished. Northernmost records (39 degrees-42 degrees S latitude) are characterized by enhanced opal burial during glacial periods and strong 41 kyr periodicity. Records from cores just north of the present Antarctic Polar Front (46 degrees-49 degrees S) show even larger increases in opal burial rate during glacial intervals, but have variance concentrated in the 100 and 23 kyr bands. Southern most records (51 degrees-55 degrees S) are completely out of phase with those to the north, with greatly reduced opal burial rates during glacial periods. Taken as a whole, the opal records show no evidence for the increased total Antarctic productivity predicted by recent geochemical models of atmospheric CO2 variability. T he areal expansion of Southern Ocean sea ice over the present zone of high siliceous productivity provides one plausible explanation for the glacial-interglacial opal patterns. The excess silica not taken up in this zone during glacial periods would contribute to greater nutrient availability and thus higher productivity in the subantarctic region. However, local circulation changes may act to modify this basic signal, possibly accounting for the observed differences in the opal variance spectra.

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.

Charles, CD, Rind D, Jouzel J, Koster RD, Fairbanks RG.  1994.  Glacial-Interglacial Changes in Moisture Sources for Greenland - Influences on the Ice Core Record of Climate. Science. 263:508-511.   10.1126/science.263.5146.508   AbstractWebsite

Large, abrupt shifts in the O-18/O-16 ratio found in Greenland ice must reflect real features of the climate system variability. These isotopic shifts can be viewed as a result of air temperature fluctuations, but determination of the cause of the changes-the most crucial issue for future climate concerns-requires a detailed understanding of the controls on isotopes in precipitation. Results from general circulation model experiments suggest that the sources of Greenland precipitation varied with different climate states, allowing dynamic atmospheric mechanisms for influencing the ice core isotope shifts.

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.

Charles, CD, Rind D, Healy R, Webb R.  2001.  Tropical cooling and the isotopic composition of precipitation in general circulation model simulations of the ice age climate. Climate Dynamics. 17:489-502.   10.1007/s003820000126   AbstractWebsite

We test the climate effects of changes in the tropical ocean by imposing three different patterns of tropical SSTs in ice age general circulation model simulations that include water source tracers and water isotope tracers. The continental air temperature and hydrological cycle response in these simulations is substantial and should be directly comparable to the paleoclimatic record. With tropical cooling imposed, there is a strong temperature response in mid- to high-latitudes resulting from changes in sea ice and disturbance of the planetary waves; the results suggest that tropical/subtropical ocean cooling leads to significant dynamical and radiative feedbacks that might amplify ice age cycles, The isotopes in precipitation generally follow the temperature response at higher latitudes, but regional delta O-18/air temperature scaling factors differ greatly among the experiments. In low-latitudes, continental surface temperatures decrease congruently with the adjacent SSTs in the cooling experiments. Assuming CLIMAP SSTs, O-18/O-16 ratios in low-latitude precipitation show no change from modern values. However, the experiments with additional cooling of SSTs produce much lower tropical continental delta O-18 values, and these low values result primarily from an enhanced recycling of continental moisture (as marine evaporation is reduced). The water isotopes are especially sensitive to continental aridity, suggesting that they represent an effective tracer of the extent of tropical cooling and drying. Only one of the tropical cooling simulations produces generalized low-latitude aridity. These results demonstrate that the geographic pattern of cooling is most critical for promoting much drier continents, and they underscore the need for accurate reconstructions of SST gradients in the ice age ocean.

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.

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.

Charles, CD, Fairbanks R.  1990.  Glacial to interglacial changes in the isotopic gradients of southern ocean surface water. The Geologic History of Polar oceans: Arctic Vs Antarctic NATO/ASI series. ( Bleil U, Thiede J, Eds.). Abstract
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Charles, CD, Hunter DE, Fairbanks RG.  1997.  Interaction between the ENSO and the Asian monsoon in a coral record of tropical climate. Science. 277:925-928.   10.1126/science.277.5328.925   AbstractWebsite

The oxygen isotopic composition of a banded coral from the western equatorial Indian Ocean provides a 150-year-long history of the relation between the El Nino-Southern Oscillation (ENSO) phenomenon and the Asian monsoon. Interannual cycles in the coral time series were found to correlate with Pacific coral and instrumental climate records, suggesting a consistent linkage across ocean basins, despite the changing frequency and amplitude of the ENSO. However, decadal variability that is characteristic of the monsoon system also dominates the coral record, which implies important interactions between tropical and midlatitude climate variability. One prominent manifestation of this interaction is the strong amplitude modulation of the quasi-biennial cycle.

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.

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.

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.

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Fairbanks, RG, Evans MN, Rubenstone JL, Mortlock RA, Broad K, Moore MD, Charles CD.  1997.  Evaluating climate indices and their geochemical proxies measured in corals. Coral Reefs. 16:S93-S100.   10.1007/s003380050245   AbstractWebsite

Standard ocean/climate indices such as the Nino-3 sea surface temperature (SST) index, based on sparse instrumental data, and atmospheric indices such as the Southern Oscillation Index (SOI), may now be substituted and/or extended by coral-based indices. Several elements or their isotopes are incorporated in coral aragonitic skeletons at predictable concentrations, some of which are temperature or salinity dependent. The availability of century-old corals, at key oceanographic sites, permits the establishment of a network of proxy climate indices.

Fairbanks, R, Charles CD, Wright JD.  1992.  Origin of the global meltwater pulses. Four decades of radiocarbon: an interdisciplinary approach. ( Long A, Kra R, Eds.).:473-500., New York, NY: Springer-Verlag Abstract
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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.

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.