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doi:10.2204/iodp.proc.303306.205.2009

Astronomical age of bioevents

In Figure F3 we show the position of the bioevents defined in this study with respect to the magnetostratigraphic and lightness records measured at Site U1313 (Expedition 306 Scientists, 2005). The lightness record was correlated by Stein et al. (2006) to the global benthic oxygen isotope stack of Lisiecki and Raymo (2005), assuming that increasing lightness, which reflects higher carbonate values, correlates with interglacial isotope stages. In this way, we accurately correlated our events with the global isotope timescale.

Most of these events have been previously recorded in Mediterranean rhythmic sections and were calibrated with the geomagnetic polarity timescale and astronomical polarity timescale (Lourens et al., 1996a, 1996b, 1998). By comparing our records with those of the Mediterranean we were able to analyze the timing of these events at the site’s latitude with the ages derived from the astronomically dated Mediterranean sections.

FO of Globorotalia puncticulata

This event was reported in the Mediterranean during insolation cycle (i-cycle) 435 (Lourens et al., 1996a) with an age of 4.52 Ma. It occurred two i-cycles below the top of the Nunivak Chron (i-cycle 433). At Site U1313 this event is located at a depth very close to the top of the Nunivak Chron (~209 mcd) and is related by Lourens et al. (1996a) with isotope Stage N1–N2.

LO of Globorotalia margaritae

This bioevent is located in the Mediterranean within the upper part of the Gilbert Chron at i-cycle 367 with an age of 3.81 Ma (Lourens et al., 1996a) and was correlated with isotope Stages Gi16–Gi15. At Site U1313 we observed this event toward the middle part of Chron Gilbert. According to preliminary correlations between the lightness record from Site U1313 and the global oxygen isotope stack (Lisiecki and Raymo, 2005), the LO of G. margaritae at Site U1313 is located at a depth very close to the position of isotope Stage Gi16, indicating that this event can be isochronous between the Mediterranean and the North Atlantic at the latitude of this site.

Disappearance of Globorotalia puncticulata

This event has been observed in the Mediterranean during i-cycle 344 with an age of 3.57 Ma, only two i-cycles above the Gilbert/Gauss Chron boundary. This reversal is recorded at Site U1313 at ~170 mcd, between 1 and 5 m below the location of this bioevent. According to preliminary correlations with the global oxygen isotope stack, this event is located at Site U1313 within the interval range of isotope Stages MG7–MG11 (Fig. F3). This bioevent was correlated with isotope Stage MG8 by Lourens et al. (1996a). Although the low resolution of our study for this part of the record does not allow an accurate location, our preliminary data suggest this event seems to be isochronous between the Mediterranean and Site U1313.

Reappearance of Globorotalia puncticulata

This event was astronomically dated in the Mediterranean at 3.31 Ma and corresponding to i-cycle 317, which is two i-cycles above the bottom of the Mammoth Chron (i-cycle 319). This corresponds to isotope Stage M2–M3 according to Lourens et al. (1996a). The event’s location at Site U1313 was observed immediately above the bottom of the Mammoth Chron and coinciding with prominent glacial isotope Stage M2 (Fig. F3). This demonstrates that this event is synchronous between the Mediterranean and the North Atlantic at mid-latitudes.

LO of Globorotalia puncticulata

The astronomical age for this event in the Mediterranean is 2.41 Ma. It occurred during i-cycle 230, which is well above the Gauss/Matuyama Chron boundary. In this study we observed this event at the base of isotope Stage 95 (Fig. F3) according to the age model of Stein et al. (2006). Its age, based on the correlation to Lisiecki and Raymo (2005), is 2.418 Ma, which is exactly the same age as that reported for the Mediterranean. Furthermore, Lourens et al. (1996a) already related this event to isotope Stage 95.

FAO of Globorotalia inflata

As shown in Figure F3, this event is related to isotope Stage 78 based on the age model proposed by Stein et al. (2006). This event was dated at 2.09 Ma by Lourens et al. (1996a) and related to i-cycle 203, which is three i-cycles above the top of the Reunion Chron. The FO of G. inflata in the Mediterranean is correlated with the FAO of the species at Site U1313 and was related with exactly the same isotope stage.

FAO of Neogloboquadrina pachyderma sinistral

This bioevent at Site U1313 lies within isotope Stages 63–64 (Fig. F3). In the Mediterranean it was reported during i-cycle 175 and was correlated with isotope Stage 64 with an age of 1.8 Ma (Lourens et al., 1996a). It occurred during the Olduvai Chron, only one i-cycle below the top of the chron. This reversal at Site U1313 is at 82.82 mcd, ~1 m above the FAO of N. pachyderma sinistral, which indicates that this event is located in the uppermost part of the Olduvai Chron.

Reduction of Neogloboquadrina pachyderma sinistral

Previous studies identified an interval in the mid-Pleistocene with very low abundances of N. pachyderma sinistral both in the Mediterranean (Lourens et al., 1996a, 1996b, 1998) and the North Atlantic (Raymo et al., 1989). This species, which usually peaks during glacial isotope stages, drastically reduced in abundance during glacial isotope Stages 42, 40, and 38 in the Vrica/Crotone composite section in Italy and Deep Sea Drilling Project Site 607, which was drilled at the same location as Site U1313. This interval was also observed at Ocean Drilling Program Site 967 and Hole 969D (Lourens et al., 1998). We identified the last peak in abundance of this species during isotope Stage 46 (Fig. F3) but did not recognize the peak of isotope Stage 44, probably because of the low resolution of our study. However, high-resolution studies at Site 607 (Raymo et al., 1989) show a peak in abundance of N. pachyderma sinistral in isotope Stage 44, whereas the species is almost absent during glacial Stages 42, 40, and 38. The reduction of N. pachyderma sinistral was dated in the Mediterranean with an age of 1.37 Ma (Lourens et al., 1996a) and was correlated with i-cycle 131.

Increase of Neogloboquadrina pachyderma sinistral

The interval with low percentages of N. pachyderma sinistral ends with a sudden increase in abundance of this species during glacial isotope Stage 36 (Fig. F3). This was observed in the Mediterranean and the North Atlantic (Lourens et al., 1996a, 1996b, 1998; Raymo et al., 1989). We recognized this event at Site U1313 near the bottom of the Cobb Mountain Subchron during isotope Stage 36, with an estimated age of 1.21 Ma based on the correlation to the Lisiecki and Raymo (2005) isotope stack. It was astronomically dated in the Mediterranean with an age of 1.22 Ma and correlated with i-cycle 117.

FO of Globorotalia truncatulinoides

We observed this event within isotope Stage 77, ~2–3 m below the bottom of the Olduvai Chron (Fig. F3). The age of this event, according to the age model of Stein et al. (2006), is 2.03 Ma. This event was not recognized in the Mediterranean.

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