Chapter II: Reconnaissance dating of deep-sea corals to develop a compre-
2.4 Discussion
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A B C
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Figure 2.2: Comparison of Southern Ocean radiocarbon reconnaissance dates and precise U/Th ages. A) U/Th ages versus IntCal13-derived calendar ages, B) U/Th ages versus Marine13-derived calendar ages, and C) U/Th ages versus Marine13- derived ages with an additional reservoir age correction of 400 yr. D, E, and F are enlargements of boxed regions in A, B, and C, respectively. In all panels blue squares are dates from this study following the method of Bush et al. (2013) and black filled diamonds are dates from Thiagarajan, Gerlach, et al. (2013) following the method of Burke, Laura F. Robinson, et al. (2010). U/Th ages are reported in Chapter 3. Gray bars mark inflection in U/Th-age14C-age relationship at∼14 ka.
seems that the Marine13-derived reconnaissance ages with the additional reservoir correction are best for selecting samples to more precisely U/Th date in the∼10–30 kyr age range. This is especially true for the Southern Ocean samples, where surface reservoir ages tend to be higher than in the North Atlantic.
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Figure 2.3: Comparison of North Atlantic radiocarbon reconnaissance dates and precise U/Th ages. A) U/Th ages versus IntCal13-derived calendar ages, B) U/Th ages versus Marine13-derived calendar ages, and C) U/Th ages versus Marine13- derived ages with an additional reservoir age correction of 400 yr. In both panels blue squares are dates from this study following the method of Bush et al. (2013) and black filled diamonds are dates from Thiagarajan, Gerlach, et al. (2013) following the method of Burke, Laura F. Robinson, et al. (2010). One sample (blue unfilled square) fell off the 1:1 line but has extremely large error bars (±95,000 yr on an age of 11,500 yr) due to high232Th (470,000 ppt). U/Th ages are reported in Chapter 5.
of the aragonite mineral. If more CO2 was stored in the deep ocean during the late glacial period (as has been hypothesized by many, e.g. Broecker, 1982; Sigman and Boyle, 2000; Sigman, Hain, and Haug, 2010), this should drive a decrease in deep ocean [CO23−], assuming the rest of the system stays the same. Indeed, Yu, Elderfield, and Piotrowski (2008) observe decreases in deep ocean [CO23−] compared to the Holocene in the high-latitude North Atlantic, and there is some evidence for decreases in intermediate water [CO23−] in the Indo-Pacific (Allen et al., 2015). There is also evidence for changes in ocean circulation configuration, with shoaled North Atlantic Deep Water and an expansion of Antarctic Bottom Water in the Atlantic bains (Curry and Oppo, 2005; Lund, Adkins, and Ferrari, 2011;
L.C. Skinner et al., 2017). This would have a much larger effect on the North Atlantic samples, because there is already a large fraction of southern-source water in the region south of Tasmania. In the modern ocean near Tasmania and the New England and Corner Rise Seamounts, much of the water between∼1000–2000 m is very close to the aragonite saturation horizon (Thiagarajan, Gerlach, et al., 2013), therefore even small changes in [CO23−] could drive the water to undersaturation. At the end of the LGM around 18 ka, atmospheric CO2begins to rise (Marcott et al., 2014). There is evidence that this CO2rise was driven by the degassing of the lower cell (Anderson et al., 2009; Burke and L F Robinson, 2012), which would lead to a deepening of the aragonite saturation horizon again.
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Shoaling of Ωarag.
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Figure 2.4: Annotated age-depth plot for Southern Ocean (A) and North Atlantic (B). Ages for all samples have been determined using the Marine13 radiocarbon calibration curve (Reimer, Bard, et al., 2013) with an additional reservoir correction of 400 years. Diamonds are from Thiagarajan, Gerlach, et al. (2013) and squares are from this study. Boxes marking the locations of the modern OMZs are based on hydrographic data (Thiagarajan, Gerlach, et al., 2013).
Another pattern that becomes clearer with the addition of new data presented here is the potential shift in the oxygen minimum zone (Figure 2.4). In the Southern Ocean, the OMZ is also associated with the return flow of old Pacific Deep Water (see Figure 3.1). At the LGM, if circulation switched such that NADW was shoaled and the upper and lower circulation cells were less intertwined, then the core of Pacific Deep Water should deepen relative to its modern arrangement in order for the lower cell to close on itself (Ferrari et al., 2014; Talley, 2013). This type of circulation configuration change could explain the presence of corals at the depth of the modern OMZ at other points during the late glacial. A two-cell circulation configuration, and thus a deepening of Pacific Deep Water return flow does not explain a shoaled OMZ during the late glacial. If the OMZ near Tasmania did shift, it is difficult to explain why this might have occurred. In general, as Thiagarajan, Gerlach, et al. (2013) note, the LGM ocean should be more oxygenated due to the temperature dependence of O2solubility. Changes in the biological productivity of surface waters could increase or decrease the intensity of an OMZ, but they shouldn’t change its depth. That leaves changes in circulation as the most likely explanation, but it is difficult to imagine what these changes might have been.
There is potentially more information that can be gleaned from the radiocarbon age- U/Th age plot as well (Figure 2.2). As was discussed above, the radiocarbon-derived calendar ages depend on both the ventilation age of the water (the amount of time that has elapsed since the water was at the surface) and the surface reservoir age (the depletion in surface water radiocarbon relative to the atmosphere). Southern Ocean reservoir ages tend to be old, because the amount of time that water typically spends at the surface is shorter than the isotopic equilibration time (O(10) years). During the LGM, Southern Ocean reservoir ages were thought to be much older because the deep ocean was more depleted in radiocarbon and expanded sea ice could have further impeded isotopic equilibration with the atmosphere (L Skinner et al., 2015;
Sikes et al., 2000). If we look at the bottom panels of Figure 2.2, there seems to be a slight inflection in the data at around 14 ka, marked with gray bars. This coincides with the Bølling-Allerød/Antarctic Cold Reversal, a time of resumed North Atlantic Deep Water formation and warm temperatures in the Northern Hemisphere and cooling in the Southern Hemisphere. It is also a time when circulation appears to have shifted towards a more modern-like configuration (Thiagarajan, Subhas, et al., 2014) and much of the CO2-rich, radiocarbon depleted LGM water has been ventilated (Burke and L F Robinson, 2012). It would make sense, therefore, that this time period would be associated with a shift toward younger surface reservoir
21 ages in the Southern Ocean.