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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.

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.

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.