Publications

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2013
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.

2010
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.

2006
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.