Abstract
Abstract. Changes in the geometry of ocean meridional overturning circulation (MOC) are crucial in controlling past changes of climate and the carbon inventory of the atmosphere. However, the accurate timing and global correlation of short-term glacial-to-deglacial changes of MOC in different ocean basins still present a major challenge. The fine structure of jumps and plateaus in atmospheric and planktic radiocarbon (14C) concentration reflects changes in atmospheric 14C production, ocean–atmosphere 14C exchange, and ocean mixing. Plateau boundaries in the atmospheric 14C record of Lake Suigetsu, now tied to Hulu Cave U∕Th model ages instead of optical varve counts, provide a stratigraphic “rung ladder” of up to 30 age tie points from 29 to 10 cal ka for accurate dating of planktic oceanic 14C records. The age differences between contemporary planktic and atmospheric 14C plateaus record the global distribution of 14C reservoir ages for surface waters of the Last Glacial Maximum (LGM) and deglacial Heinrich Stadial 1 (HS-1), as documented in 19 and 20 planktic 14C records, respectively. Elevated and variable reservoir ages mark both upwelling regions and high-latitude sites covered by sea ice and/or meltwater. 14C ventilation ages of LGM deep waters reveal opposed geometries of Atlantic and Pacific MOC. Like today, Atlantic deep-water formation went along with an estuarine inflow of old abyssal waters from the Southern Ocean up to the northern North Pacific and an outflow of upper deep waters. During early HS-1, 14C ventilation ages suggest a reversed MOC and ∼1500-year flushing of the deep North Pacific up to the South China Sea, when estuarine circulation geometry marked the North Atlantic, gradually starting near 19 ka. High 14C ventilation ages of LGM deep waters reflect a major drawdown of carbon from the atmosphere. The subsequent major deglacial age drop reflects changes in MOC accompanied by massive carbon releases to the atmosphere as recorded in Antarctic ice cores. These new features of MOC and the carbon cycle provide detailed evidence in space and time to test and refine ocean models that, in part because of insufficient spatial model resolution and reference data, still poorly reproduce our data sets.
Highlights
1.1 A variety of terms linked to the notion “14C age”The 14C concentration in the troposphere is mainly determined by 14C production, atmospheric mixing, air–sea gas exchange, and ocean circulation, which vary over time (e.g., Alves et al, 2018; Alveson, 2018)
Apart from U/Th dated corals, the 14C age of planktic foraminifers is the most common tracer in marine sediments, providing a rough estimate of the time passed since sediment deposition
In view of the recent revision of timescales (Schlolaut et al, 2018; Bronk Ramsey et al, 2020), we extended our plateau tuning and defined the boundaries and age ranges of 14C plateaus and jumps for the interval ∼ 23– cal ka, which results in a total of ∼ atmospheric age tie points for the time span 10.5–29 cal ka (Fig. 1; summary in Table 1; following the rules of Sarnthein et al, 2007, 2015)
Summary
The 14C concentration in the troposphere is mainly determined by 14C production, atmospheric mixing, air–sea gas exchange, and ocean circulation, which vary over time (e.g., Alves et al, 2018; Alveson, 2018). Obtaining such tie points presents a problem, since any attempt to date a deep-sea sediment record by means of 14C encounters a number of intricacies of how to disentangle the effects of global atmospheric 14C variations due to past changes in cosmogenic 14C production and carbon cycle from (i) local depositional effects such as sediment hiatuses and winnowing, differential bioturbational mixing depths, and sediment transport by deep burrows; (ii) the effects of local atmosphere–ocean exchange and ocean mixing, resulting in reservoir and ventilation ages that change through time and space (e.g., Alves et al, 2018; Grootes and Sarnthein, 2006); and (iii) quantitatively “pure” 14C ages due to radioactive decay from the final target These problems are exacerbated by the need for a generally accepted high-precision atmospheric reference record for the period 14–50 cal ka, which is beyond tree ring calibration. This suite of tie points may facilitate a precise temporal correlation of all sorts of changes in surface and deep-water composition on a global scale, crucial for a better understanding of past changes in ocean and climate dynamics
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