Bibliography - B. G. Brunelle

1. Brunelle, B. G., Daniel Sigman, S. L. Jaccard, L. D. Keigwin, B. Plessen, G. Schettler, M. S. Cook, and G. H. Haug, 2010: Glacial/interglacial changes in nutrient supply and stratification in the western. Quaternary Science Reviews, Elsevier, (29), doi:10.1016/j.quascirev.2010.03.010 2579-2590
   [ Abstract ]

   In piston cores from the open subarctic Pacific and the Okhotsk Sea, diatom-bound δ¹⁵N (δ¹⁵N_db), biogenic opal, calcium carbonate, and barium were measured from coretop to the previous glacial maximum (MIS 6). Glacial intervals are generally characterized by high δ¹⁵N_db (w8&) and low productivity, whereas interglacial intervals have a lower δ¹⁵N_db (5.7-6.3 per mil) and indicate high biogenic productivity. These data extend the regional swath of evidence for nearly complete surface nutrient utilization during glacial maxima, consistent with stronger upper water column stratification throughout the subarctic region during colder intervals. An early deglacial decline in δ¹⁵N_db of 2 per mil at ~17.5 ka, previously observed in the Bering Sea, is found here in the open subarctic Pacific record and arguably also in the Okhotsk, and a case can be made that a similar decrease in δ¹⁵N_db occurred in both regions at the previous deglaciation as well. The early deglacial δ¹⁵N_db decrease, best explained by a decrease in surface nutrient utilization, appears synchronous with southern hemisphere-associated deglacial changes and with the Heinrich 1 event in the North Atlantic. This δ¹⁵N_db decrease may signal the initial deglacial weakening in subarctic North Pacific stratification and/or a deglacial increase in shallow subsurface nitrate concentration. If the former, it would be the North Pacific analogue to the increase in vertical exchange inferred for the Southern Ocean at the time of Heinrich Event 1. In either case, the lack of any clear change in paleoproductivity proxies during this interval would seem to require an early deglacial decrease in the iron-to-nitrate ratio of subsurface nutrient supply or the predominance of light limitation of phytoplankton growth during the deglaciation prior to Bølling-Allerød warming.

2. Brunelle, B. G., Daniel Sigman, M. S. Cook, L. D. Keigwin, G. H. Haug, B. Plessen, G. Schettler, and S. L. Jaccard, 2007: Evidence from diatom-bound nitrogen isotopes for Subarctic Pacific stratification during the last ice age and a link to North Pacific denitrification changes. Paleoceanography, 22(PA1215), doi:10.1029/2005PA001205
   [ Abstract ]

   In a piston core from the central Bering Sea, diatom microfossil-bound N isotopes and the concentrations of opal, biogenic barium, calcium carbonate, and organic N are measured over the last glacial/interglacial cycle. Compared to the interglacial sections of the core, the sediments of the last ice age are characterized by 3% higher diatom-bound δ¹⁵N, 70 wt % lower opal content and 1200 ppm lower biogenic barium. Taken together and with constraints on sediment accumulation rate, these results suggest a reduced supply of nitrate to the surface due to stronger stratification of the upper water column of the Bering Sea during glacial times, with more complete nitrate consumption resulting from continued iron supply through atmospheric deposition. This finding extends the body of evidence for a pervasive link between cold climates and polar ocean stratification. In addition, we hypothesize that more complete nutrient consumption in the glacial age subarctic Pacific contributed to the previously observed ice age reduction in suboxia and denitrification in the eastern tropical North Pacific by lowering the nutrient content of the intermediate-depth water formed in the subpolar North Pacific. In the deglacial interval of the Bering Sea record, two apparent peaks in export productivity are associated with maxima in diatom-bound and bulk sediment δ¹⁵N. The high δ¹⁵N in these intervals may have resulted from greater surface nutrient consumption during this period. However, the synchroneity of the deglacial peaks in the Bering Sea with similar bulk sediment δ¹⁵N changes in the eastern Pacific margin and the presence of sediment lamination within the Bering Sea during the deposition of the productivity peaks raise the possibility that both regional and local denitrification worked to raise the δ¹⁵N of the nitrate feeding Bering Sea surface waters at these times.

3. Robinson, R. S., B. G. Brunelle, and Daniel Sigman, 2004: Revisiting nutrient utilization in the glacial Antarctic: Evidence from a new method for diatom-bound N isotopic analysis. Paleoceanography, 19(PA3001), doi:10.1029/2003PA000996
   [ Abstract ]

   Isotopic measurements of diatom-bound nitrogen, using a wet chemical oxidation combined with the ‘‘denitrifier’’ method for nitrate analysis, show significant offsets from previously published combustion-based measurements. This offset is attributed to a gaseous nitrogen blank associated with the diatom’s opal frustule. Moreover, experimentation with multiple chemical cleaning protocols demonstrates that diatom microfossils from the clay-rich sediments of the glacial Antarctic are more difficult to clean than Holocene materials. New downcore profiles from the Antarctic show no change in the diatom-bound N ¹⁵N/¹⁴N between the last glacial and the Holocene in the Atlantic sector, and the elevation of glacial diatom-bound ¹⁵N/¹⁴N relative to the Holocene in the Indian sector is smaller than in previous measurements. These data suggest no change in the degree of nitrate utilization in the Atlantic sector and at most a 20% increase (from ~ 25 to 45%) in the Indian sector. The new measurements suggest that, during the last ice age in the Atlantic sector of the Antarctic, the atmospheric source of biologically available iron was not so great as to become significant relative to the iron supply from below. Given the apparent spatial variability in the degree of nitrate drawdown, more work is required to develop an adequate picture of the glacial Antarctic nutrient field.

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