Previous   |    Next

doi:10.2204/iodp.proc.303306.216.2013

Results and discussion

The G. bulloides δ¹⁸O data between the core top and 9.4 meters composite depth (mcd) below seafloor show that this interval contains climate records from MISs 1–6. G. bulloides δ¹⁸O values range between 0.35‰ and 3.02‰, in which lower (or more depleted) values were associated with the interglaciation or interstadial periods. In contrast, higher (or more enriched) values were correlated to the glacial or stadial periods (Fig. F1). We resolved major climate transitions of the last glacial–interglacial cycle, including the Terminations TI and TII at 0.65 and 6.45 mcd, respectively. Abundances of N. pachyderma (s) vary from 0% to 18%, and high values are always associated with Heinrich iceberg-rafting events in this region (Chapman et al., 2000; Rashid and Boyle, 2007; Naafs et al., 2011). See Tables T1 and T2 for quantitative data.

Abundances of four planktonic foraminifers and IRD/g data from 6 to 9.5 mcd are plotted in Figure F2. This depth range covers the climate interval from MISs 5e to 6 (Fig. F3A) (see the “Site U1313” chapter [Expedition 306 Scientists, 2006]). High-resolution G. bulloides δ¹⁸O data show a typical deglacial sequence from MISs 5e to 6 in which higher δ¹⁸O values were found in glacial MIS 6 and lower values were associated with MIS 5e (Fig. F3B). The gradual change from higher to lower δ¹⁸O values between the isotope stages was identified at Termination TII (Fig. F3B). More positive G. bulloides δ¹⁸O values (Fig. F2) were found in MIS 6 compared to MIS 5e, and δ¹⁸O values progressively became more positive from 194 to 156 ka (i.e., from 9.21 to 7.32 mcd). The most positive δ¹⁸O values were obtained between 156 and 132 ka (i.e., between 7.32 and 6.55 mcd) during the coldest part of MIS 6. The nature of the δ¹⁸O curve shows a more gradual transition at Termination TII compared to Termination TI at Site U1313, in which a rapid transition from glacial to interglacial state is observed.

Four IRD peaks were identified in MIS 6 (Fig. F2). Event H11 was identified halfway through Termination TII. However, the mean IRD/g in some peaks of MIS 6, such as at 8.40 mcd, was higher than Event H11 (Ruddiman, 1977; Chapman and Shackleton, 1998). The four IRD/g peaks in MIS 6 at Site U1313 are consistent with the high-resolution records from the southern Portuguese margin (Margari et al., 2010) and correspond to more negative δ¹⁸O values of planktonic foraminifers suggestive of sea-surface salinity dilution.

N. pachyderma (s) (Fig. F2) remains low in abundance (<5%) throughout MIS 6 and rapidly decreases at the onset of Termination TII. It reaches a maximum of 10% abundance at the end (131–130.4 ka) of Event H11, thus doubling its highest glacial abundance. G. bulloides maintains a variable presence throughout MIS 6, reaching a maximum of nearly 40% abundance during Event H11 with a broad peak centered at 160 ka. The abundance curves of G. bulloides and N. pachyderma (s) generally follow each other. The subtropical location of Site U1313 is normally unsuitable for N. pachyderma (s) to dwell under modern climate conditions (Fairbanks et al., 1980), but the cold conditions during IRD events created a suitable environment for this species.

The baseline concentration of N. incompta hovers at 5% throughout MIS 5e to 6, with four dominant peaks reaching as high as 26% with two other minor peaks. It seems that the overall concentration of N. incompta is anticorrelated with the abundances of G. bulloides and N. pachyderma (s), which is consistent with the findings of Chapman at al. (2000) from nearby Core SU90-03 (Fig. F2). However, N. incompta concentration shows positive correlation with G. inflata during the latter part of MIS 6 (6.65–6.97 mcd).

G. inflata (Fig. F2) increases in late MIS 6 (155–134 ka) to nearly 14%, compared to <2% abundance from the earlier part of MIS 6. At the onset of Event H11, it decreases abruptly and reaches a minimum of ~4%. At the end of Event H11, it increases rapidly, reaching close to 16% abundance in MIS 5e. G. inflata exists in low abundance at Site U1313 throughout MIS 6.

Distinct changes in N. pachyderma (s), N. incompta, G. bulloides, and G. inflata abundances were found during Event H11. This event could have been associated with freshening and/or temporary cooling of the sea surface. Sea-surface cooling is substantiated by the presence of polar water mass indicator species N. pachyderma (s) (Bond et al., 1993) and low abundances of N. incompta. On the other hand, the high abundance of G. bulloides may indicate limited influence of polar water, which is consistent with the relatively low abundances of N. pachyderma (s). If the surface water was totally of polar to subpolar origin, then N. incompta should have also peaked in abundance. The high abundance of G. bulloides (a species that is very much food driven) may indicate that there was a mixture of polar and transitional water influencing Site U1313, with a warmer sea surface during Event H11 similar to the younger Heinrich iceberg-rafting events as observed by Naafs et al. (2013). This was further demonstrated by the sharp decline of temperate species G. inflata (Kucera et al., 2005). The warmer sea-surface conditions of MIS 5e were marked by the increase of G. inflata and subsequent decline in G. bulloides and N. incompta.

The high sensitivity of N. pachyderma (s) to Heinrich iceberg-rafting events was attributed to its polar water habitat (Bond et al., 1993). The peaks in N. pachyderma (s), N. incompta, and G. bulloides at Site U1313 suggest that the freshwater event (H11) cooled the sea surface, thus allowing these species to reach a temporary peak in abundance. At Site U1313, higher percentages of G. bulloides may coincide with higher availability of food.

Top of page   |    Previous   |    Next