6 Science Highlights : U . S . ESH Program Climate-related changes in export production off the Pacifi c coast of Mexico during the last deglaciation

Climate-related changes in the ocean’s oxygen minimum zones A striking feature of abrupt climate change during the last glacial period is the correlation between temperature proxy records from Greenland ice cores and the intensity of the oxygen minimum zone (OMZ) in the eastern North Pacifi c Ocean (Ortiz et al., 2004) and the Arabian Sea (Altabet et al., 2002). Oxygen concentrations decreased during warm interstadial periods, much like conditions today, whereas oxygen concentrations were much greater during cold stadials and the Last Glacial Maximum. Intensifi cation of the OMZ is manifested in several proxies, including concentrations in sediments of organic carbon and of redox-sensitive trace elements, as well as in nitrogen isotope indicators of denitrifi cation. Various investigators have attributed these features to either 1) changes in the supply of oxygen associated with the ventilation of intermediate water mass, or 2) local changes in productivity, as a result of ocean-atmosphere reorganizations, such as is seen during El Niño events today in the eastern North Pacifi c. Whereas evidence for changes in the intensity of the OMZ is unequivocal, most sedimentary proxies cannot discriminate between the two proposed causes because the impact of low bottomwater oxygen concentrations on the composition of marine sediments (e.g., carbon concentration; trace metal enrichment) is generally indistinguishable from that of high biological productivity and associated organic rain rates.


Climate-related changes in the ocean's oxygen minimum zones
A striking feature of abrupt climate change during the last glacial period is the correlation between temperature proxy records from Greenland ice cores and the intensity of the oxygen minimum zone (OMZ) in the eastern North Pacifi c Ocean (Ortiz et al., 2004) and the Arabian Sea (Altabet et al., 2002).Oxygen concentrations decreased during warm interstadial periods, much like conditions today, whereas oxygen concentrations were much greater during cold stadials and the Last Glacial Maximum.Intensifi cation of the OMZ is manifested in several proxies, including concentrations in sediments of organic carbon and of redox-sensitive trace elements, as well as in nitrogen isotope indicators of denitrifi cation.
Various investigators have attributed these features to either 1) changes in the supply of oxygen associated with the ventilation of intermediate water mass, or 2) local changes in productivity, as a result of ocean-atmosphere reorganizations, such as is seen during El Niño events today in the eastern North Pacifi c.Whereas evidence for changes in the intensity of the OMZ is unequivocal, most sedimentary proxies cannot discriminate between the two proposed causes because the impact of low bottomwater oxygen concentrations on the composition of marine sediments (e.g., carbon concentration; trace metal enrichment) is generally in-distinguishable from that of high biological productivity and associated organic rain rates.

New geochemical proxies link OMZ changes to biological productivity
Natural radionuclides can help resolve this.Due to the differential solubilities of the radionuclides, 231 Pa/ 230 Th and 10 Be/ 230 Th ratios increase with increasing particle fl ux (Kumar et al., 1995), which, in turn, is regulated by biological productivity.These ratios have the advantage that they are insensitive to the concentration of dissolved oxygen.However, they are sensitive to changes in particle composition (e.g., the ratio of clay to biogenic particles or the ratio of carbonate to opal (Chase et al., 2002)) as well as to changes in particle fl ux (productivity).Therefore, although these ratios do not provide a unique and unambiguous paleoproductivity tracer, they are a valuable component of a multiproxy study because their sensitivities to changing environmental parameters are orthogonal to the sensitivities of other paleoproductivity proxies.

. ( ) Decay-corrected unsupported D) Decay-corrected unsupported D
231 Pa / 230 Th (activity ratio; diamonds) and 10 Be / 230 Th (10 9 atoms/dpm, divided by 2 to put them on the same scale as Pa/Th; circles). 9atoms/dpm, divided by 2 to put them on the same scale as Pa/Th; circles).Science Highlights: U.S. ESH Program 2004) of opal and of organic carbon were positively correlated with the high organic carbon content of sediments deposited during warm periods (Bølling and Holocene; Fig. 1).We fi nd elevated 231 Pa/ 230 Th and 10 Be/ 230 Th ratios during warm periods as well (Fig. 1), indicating that increased productivity must have contributed to the higher preserved fl uxes of opal and carbon at those times.

Conclusions
While these results do not rule out changes in the ventilation of intermediate waters, they do provide clear evidence for changes in bio-logical productivity and the export fl ux of biogenic particles.Enhanced productivity during warm events refl ects a shoaling of the nutricline, an increase in upwelling-favorable winds, or some combination thereof.

Introduction
One of the most cited papers (271 citations by June 2006) on Holocene climate change is the 2001 paper by Bond and colleagues (2001), which argued for a pervasive ~1500 yr signal in the delivery of hematitestained quartz (HSQ) sands to sites at or beyond the historic limits of observed drift ice (Fig. 1) (we use the term "drift ice" to denote any mixture of glacier icebergs or various forms of sea ice).To critically examine the hypotheses set forth in Bond et al. (2001), we selected a series of cores for analysis of ice-rafted debris (IRD).These cores were collected from sites that are much more within the historically known limits of drift ice (Fig. 1), hence lie to the north of the sites examined for HSQ.Two of the cores are exceptionally well dated (Moros et al., 2006) and the other two have suffi cient age control to examine the Holocene trends in IRD.

Methods
There

Figure 1 :
Figure 1: Location of cores used in this study (red circles) and cores reported by Bond et al. (2001).Historic maximum limits of drift ice are shown based on data from the International Ice Patrol (www.uscg.mil/LANTAREA/IIP/home.html) and the Norsk Polar Institute (acsys.npolar.no/ahica/quicklooks/looks.htm).Arrow labels: EGC = East Greenland Current; B/LC = Baffi nland and Labrador Current; EIC = East Iceland Current.