Ocean thermohaline circulation and sedimentary231Pa/230Th ratio
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Holocene sediments from the Atlantic are characterized by Pa-231/Th-230 ratios below the production ratio of the two radionuclides in the water column (0.093), whereas Holocene sediments from the Southern Ocean have Pa-231/Th-230 > 0.093. This pattern of Pa-231 deficit and excess was ascribed to southward Pa-231 export from the Atlantic by the Atlantic thermohaline circulation (THC) as Pa is scavenged less efficiently by marine particles and more effectively transported by the THC than Th. The same pattern is observed in deposits of the Last Glacial Maximum (LGM), which led to a previous contention that the THC strength did not vary markedly through the last glacial termination. Here we embed a description of trace metal scavenging into a zonally averaged, circulation-biogeochemistry ocean model to explore the sensitivity of Pa-231/Th-230 in Atlantic and Southern Ocean sediments to THC changes. Our results show that the production of biogenic opal (which, unlike other marine particles, poorly fractionates Th and Pa) in the Southern Ocean water column determines the spatial pattern of the sensitivity. Also, Pa-231/Th-230 increases in the North Atlantic but changes little in the South Atlantic and decreases in the Southern Ocean as THC is reduced. The mean Pa-231/Th-230 of the whole Atlantic is therefore less sensitive to THC changes than the mean Pa-231/Th-230 Of the North Atlantic. The current uncertainties in Atlantic mean Pa-231/Th-230 are too large to rule out a two-fold reduction of the THC at the LGM. However, the increase in North Atlantic mean Pa-231/Th-230 simulated in response to a twofold THC reduction is larger than the observed change in the North Atlantic mean Pa-231/Th-230 from the LCM to Holocene. Comparing this change with the modeled sensitivity of North Atlantic Pa-231/Th-230 to THC variations indicates that the THC at the LGM could not have been reduced by > 30% of its present strength. Experiments of transient THC changes indicate that, high-resolution Pa-231/Th-230 records from North Atlantic sediments could also document thermohaline oscillations on century-to-millennial timescales.