A pervasive link between Antarctic ice core and subarctic Pacific ...

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S.L. Jaccard et al. / Quaternary Science Reviews 29 (2010) 206–212 209 amplitude (Fig. 3) of the signal. Ba/Al and, to a lesser degree, Ca/Al show a generally lower amplitude between 450 and 750 kyrs during the ‘‘luke-warm’’ interglacials, MIS 13, 15 & 17 (Figs. 3 and 4). In particular, interglacial maxima show significantly lower values when compared to the past 450 kyr, the glacial baseline remaining rather constant across the entire record (Fig. 2). We note, however, that the correlation between ODP882 Ba/Al and EDC dD and CO2 was markedly weaker prior to 450kyr, for reasons that are currently unclear. Further investigations will aim to clarify this issue. 4. Discussion Reconstruction of past changes in organic carbon export is uncertain, as each proxy has its own biases. Here we analyze two different proxies, both of which yield a picture that is internally consistent and supported by data from the literature (see above). It is important to point out that the concentrations of both organic carbon and chlorins (an algal biomarker) are higher in glacial age sediments, in contrast to the bioBa and opal (Gebhardt et al., 2008; Gorbarenko, 1996; Haug et al., 1995; Kiefer et al., 2001; Shigemitsu et al., 2007). These organic phases might seem to be more direct proxies of carbon export than the mineral proxies we present here. However, the maximum chlorin concentrations observed in core MD01-2416 (Gebhardt et al., 2008), gradually decrease downcore with values up to 10 000 ng/g for TERM I to 20 mM) (Pedersen and Ingram, 1995). We therefore assume here that the less direct, but hopefully more robust mineral proxies are the most accurate reflection of past changes in export production, with the caveat that the causes of higher glacial concentrations of organic components should be more thoroughly investigated. Shigemitsu et al. (2007) recently presented a novel approach to address past changes in the dust flux to the western subarctic Pacific. Their results show that the eolain contribution to the sediment was approximately twice as high during the past ice ages when compared to interglacials. Preliminary results, based on 232 Th MAR (Jaccard et al., 2009) seem to support their conclusion. As a result, a portion of the Ba/Al signal is likely to reflect the enhanced input of eolian material during glacial intervals. However, 230Thnormalized flux reconstructions for TERM I (Jaccard et al., 2009) clearly confirm that the last deglaciation was marked by a large increase in the preserved flux of biogenic detritus, accompanying the inferred decrease in dust supply. Reconstructions of sea surface temperature from alkenones (Haug, 1996), foraminiferal Mg/Ca (Gebhardt et al., 2008), and foraminiferal transfer functions (Kiefer et al., 2001; Kiefer and Kienast, 2005) all indicate that, even during the coldest times, summertime SST never approached freezing in the vicinity of the core site. This corroborates micropaleontological evidence that ODP Site 882 is located well east of the maximum perennial sea-ice extent over the Pleistocene (Climap Members, 1981) (Sancetta and Silvestri, 1986). As a result, sea ice cover is unlikely to have represented a major limitation on the spring/summer growing season during glacial times. Rather, some other factor must have limited phytoplankton growth during glacial times. The aeolian dust supply (cps) Ba/Al D‰ ( vsSMOW ) δ 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 -360 -380 -400 -420 -440 T V T IV T VIII T VII TI T T III T IX II T VI b d a c 200 150 100 50 0 300 280 260 240 220 200 180 160 Ca/Al (cps) C O (ppmv ) 2 -460 0 100 200 300 400 500 600 700 800 Age (kyr) Fig. 2. (a) Ca/Al and (b) Ba/Al records from ODP site 882 compared to (c) EDC CO 2 (Lüthi et al., 2008) (Monnin et al., 2001) (Siegenthaler et al., 2005) and (d) the deuterium (dD) (Jouzel et al., 2007) records during the past 800 kyrs. Glacial Terminations are indicated using Roman numerals in subscript.
210 S.L. Jaccard et al. / Quaternary Science Reviews 29 (2010) 206–212 1.5 Normalized Amplitude 1.0 0.5 CO 2 δD Ba/Al 0 T I T II T III T IV T V T VI T VII T VIII T IX Fig. 3. Interglacial-glacial CO 2 (dark grey), dD (light grey) and Ba/Al (red) amplitude for glacial terminations I-VII. Amplitude is (arbitrarily) defined as the difference between the three consecutive highest interglacial values and the three consecutive lowest values preceding the onset of glacial terminations. The amplitude of each parameter has been normalized to TERM 1 values. of iron to the region was, if anything, higher during colder times (Kienast et al., 2004; Shigemitsu et al., 2007). It is still possible that some other physical change, such as a deeper summertime mixed layer, led to greater light limitation during ice ages (e.g. Anderson et al., 2002). Alternatively, reduced upwelling or vertical mixing may have decreased the supply from below of both iron and the major nutrients. In this case, the increased supply of iron from above must have failed to compensate for the reduced supply from below, leading to the net decrease in productivity. Nevertheless, this situation should result in more complete major nutrient consumption, because of the greater proportional importance of the iron input from above. Indeed, subarctic northwest Pacific sediment and diatom-bound d 15 N – a measure of the relative utilization of dissolved nitrate (Altabet and François, 1994) – shows a strong negative correlation with export production on >5ky timescales (Brunelle et al., 2007; Galbraith et al., 2008; Nakatsuka et al., 1995)(Fig. 1 panel (a)). This suggests that, during ice ages, the relative amount of nitrate that was utilized by phytoplankton increased, even though the absolute rate of nitrate consumption decreased, which implies that the supply of nutrients to the subarctic North Pacific was reduced during ice ages. The N isotope data in themselves suggest that the efficiency of the subarctic Pacific biological pump increased during ice ages, thereby contributing to sequester a larger burden of remineralized carbon into the abyssal ocean (Brunelle et al., 2007; Galbraith et al., 2008). Enhanced glacial storage of metabolic carbon would have reduced oxygenation of the abyssal ocean, consistent with other data, such as authigenic Uranium content (Galbraith et al., 2007; Jaccard et al., 2009; Shigemitsu et al., 2007). Similarly, d 15 N records from the Antarctic Zone of the Southern Ocean indicate higher relative nitrate utilization and yet reduced export production during peak ice ages (François et al., 1997; Robinson et al., 2004; Robinson and Sigman, 2008; Schneider-Mor et al., 2005), suggesting that either a common physical mechanism modulated the nutrient supply of both these polar regions in tandem (Sigman et al., 2004), or that a physical change in the Southern Ocean controlled the subarctic Pacific nutrient budget remotely. Biogenic carbonate (Ca/Al), which was nearly or completely absent in these cores throughout much of the glacial sediments, suddenly appears in abundance at each glacial termination. Although the increase in CaCO 3 concentration may reflect enhanced local CaCO 3 export, the rapid increase suggests a rapid deepening of the lysocline, driven by a decrease of DIC and/or an increase in the alkalinity of bottom waters at times when deeply sequestered CO 2 is released to (sub)surface waters. If these intervals were solely deglacial, then they could be taken as indicative of the global preservation event that one would predict at the end of ice ages, when CO 2 is released from the ocean to accumulate in the atmosphere and terrestrial biosphere (e.g. Broecker and Peng, 1985; Marchitto et al., 2005). However, high Ca/Al tends to persist into interglacials, suggesting that at least in the early part of interglacials, the subarctic North Pacific interior had a smaller burden of regenerated dissolved inorganic carbon, due either to ventilation from the South and/or locally interglacial-glacial δD ampl. (‰ vs SMOW) 80 70 60 50 40 T III T VII T IV T V T IX 30 T VIII 20 T VI R = 0.92 10 30 40 50 60 70 80 90 100 interglacial- glacial CO 2 ampl. (ppmv) T I T II interglacial-glacial δD ampl. (‰ vs SMOW) 80 70 60 50 40 30 T VII T III T I T VIII T VI T IV T V T II T IX 20 R = 0.82 10 1.0 1.5 2.0 2.5 3.0 3.5 interglacial-glacial Ba/Al ampl . interglacial-glacial CO 2 ampl. (ppmv) 100 90 80 70 T VII T IV T V T II T III T I T IX 60 T 50 VIII 40 T R = 0.76 VI 30 1.0 1.5 2.0 2.5 3.0 3.5 interglacial-glacial Ba/Al ampl . Fig. 4. Cross-plots illustrating the correlation between (a) dD and CO 2 amplitude; (b) dD and Ba/Al amplitude; (c) Ba/Al and CO 2 amplitude. Amplitude has been determined as the difference between the three consecutive highest interglacial values and the three consecutive lowest values preceding the onset of glacial terminations. R represents the linear correlation factor between variables. Glacial Terminations are indicated using Roman numerals in subscript.
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