Chapter II: Reconnaissance dating of deep-sea corals to develop a compre-
2.3 Results
The average age error for the Burke, Laura F. Robinson, et al. (2010) method is 1.6% in the Southern Ocean and 1.5% in the North Atlantic (Thiagarajan, Gerlach, et al., 2013). The average age error for the Bush et al. (2013) method is slightly higher: 3.4% for the Southern Ocean and 3.1% for the North Atlantic. 23 of the 72 intermediate depth and 17 of the 123 deep North Atlantic corals we dated were older than the detectable limit of the method. In the Southern Ocean, 17 of the 97 intermediate depth and none of the 39 deep corals were older than the detectable limit.
New reconnaissance radiocarbon dates for the Southern Ocean (Table 2.1) and North Atlantic (Table 2.2) adhere to the basic trends observed previously by Thiagarajan, Gerlach, et al. (2013) (Figure 2.1). The depth range and abundance of the deep-
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A
B
ACR LGM
LGM HS1 YD
Figure 2.1: Age-depth plot for Southern Ocean (A) and North Atlantic (B) corals.
Diamonds are data from Thiagarajan, Gerlach, et al. (2013) and squares are from this study. Lightest colored symbols (legend far right) are calculated using the IntCal13 calibration curve, medium colored symbols (legend center) are calculated using the Marine13 calibration curve, and darkest symbols (legend far left) are calculated using the Marine13 calibration curve and an additional reservoir correction of 400 years.
sea coral population responds to regional climate, particularly rapid climate change events during the deglaciation. In the North Atlantic there are abundance peaks and depth excursions corresponding to the Younger Dryas and Heinrich Stadial 1, and in the Southern Ocean there is an abundance peak corresponding to the Antarctic Cold Reversal. Coral abundances and depth ranges are greatly reduced at the LGM compared to earlier in the glacial and later during the deglaciation.
Since many of the radiocarbon-screened deep-sea corals have since been more precisely and accurately U/Th dated, it is possible to further examine the accu- racy of reconnaissance radiocarbon age determinations by making U/Th age versus radiocarbon-derived calendar age plots. Figure 2.2 shows a comparison of Southern Ocean reconnaissance dates and U/Th ages for samples dated using the method- ology of Burke, Laura F. Robinson, et al. (2010) (black diamonds) (Thiagarajan, Gerlach, et al., 2013) and Bush et al. (2013) (blue squares). There is generally good correspondence between reconnaissance dates and U/Th dates, except for the oldest samples, where the radiocarbon-derived ages tend to underestimate the age relative to the more precise and accurate U/Th dates. This is not particularly surprising, since the oldest samples push the limit of detection for the radiocarbon methods used.
Figure 2.2 D–F are expanded versions of A–C, focusing on the interval between 10 and 30 ka. In these expanded views it is clear that reconnaissance dates using the Marine13 calibration curve with the additional 400-year reservoir correction does the best job of matching the U/Th ages.
There are fewer U/Th dates for comparison in the North Atlantic (Figure 2.3), however the general trends are consistent. The reconnaissance date underestimates the U/Th age for the oldest samples and the Marine13 calibration curve with an additional 400-year reservoir correction gives the best date, particularly for samples in the 10–30 ka age window. One sample, reconnaissance dated to ∼26 ka, gave a U/Th date of 11.5±95.0 ka (unfilled square with large x error bars in Figure 2.3). This was due to high 232Th (470,000 ppt), which increases age error due to uncertainty in the initial230Th/232Th ratio.
This comparison between U/Th and reconnaissance radiocarbon dates focuses pri- marily on the late glacial and deglaciation, where surface reservoir ages are larger than they are in the Holocene. It should also be noted that reconnaissance ages for deep-sea corals should always be slightly older than U/Th ages because they also record the ventilation age of the water (i.e. the amount of time that has elapsed since the water was at the surface in equilibrium with the atmosphere). Given this, it still
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D E F
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.