[M-IS12] Paleoclimatology and paleoceanography

[MIS12-16] Innovative advances in radiocarbon calibration methods to improve age models of Southern Ocean cores

*Minoru Ikehara¹, Stephen Obrochta² (1.Marine Core Research Institute, Kochi University, 2.Faculty of International Resource Sciences, Akita University)

Keywords:Southern Ocean, radiocarbon, reservoir ages, last glacial

The apparent radiocarbon ages of Southern Ocean (SO) surface waters are much older than in the tropics, and early studies estimated the age of modern Antarctic surface waters to be ~1,200 years (Stuiver and Polach, 1977). Furthermore, the possibility that glacial SO surface waters may have an even older radiocarbon age was proposed from core analyses off New Zealand (Sikes et al., 2000). They suggested that surface-reservoir ages from 11,900 ¹⁴C years ago were twice as large (800 years) from the tropics and during glacial times were five times as large (2,000 years) compared to present-day values. Many Southern Ocean paleoceanographers suspected that the glacial reservoir ages were older, but there was little information on glacial reservoir age available on a regional and systematic basis for the SO. Therefore, reservoir age offsets were typically not considered. A recent study estimated the reservoir ages of glacial SO surface waters at various latitudes (Heaton et al., 2023), leading to innovative advances in the construction of age models for the SO cores.
In this study, we apply the results of Heaton et al. (2023) to a marine core (COR-1bPC) from the Conrad Rise in the Indian sector of the SO and test its differences from previous age models. The lowermost part of this core is about 43,000 years old, and AMS ¹⁴C dates of planktic foraminifers have already been obtained at 23 levels. Calendar age calibration was performed with MatCal (Lougheed and Obrochta, 2016) with the Marine20 calibration curve (Heaton et al., 2020), and two different additional reservoir age correction (ΔR) scenarios to consider the effect of variable sea ice conditions in polar regions (Heaton et al., 2023). Comparison of the depositional age of COR-1bPC in the new age model with the existing age model calibrated in Marine13 shows that the last glacial to deglacial depositional age is on average about 870 years younger and up to about 1400 years younger. As a result, comparisons between the paleoceanographic proxy record of COR-1bPC and the paleoenvironmental record of the Antarctic ice core are improved to be more realistic. Many previous studies in the SO have calibrated glacial period sedimentary ages to the same reservoir ages as modern times, which means that the discussion has developed on an age axis that is several hundred to a thousand years older. Therefore, most of the discussions comparing the short-term variability of the last glacial to deglacial period in the SO with ice cores and tropics records should be re-examined.