Large variations in the Holocene marine radiocarbon reservoir effect reflect ocean circulation and climatic changes

Accurate radiocarbon dating of marine samples requires knowledge of the marine radiocarbon reservoir effect. This effect for a particular site/region is generally assumed constant through time when calibrating marine C-14 ages. However, recent studies have shown large temporal variations of several hundred to a couple of thousand years in this effect for a number of regions during the late Quaternary and Holocene. Here we report marine radiocarbon reservoir correction (Delta R) for Heron Reef and Moreton Bay in southwestern (SW) Pacific for the last 8 ka derived from C-14 analysis of Th-230-dated corals. Most of our Delta R for the last similar to 5.4 ka agree well with their modern value, but large Delta R variability of similar to 410 yr (from trough to peak) with possible decadal/centennial fluctuations is evident for the period similar to 5.4-8 ka. The latter time interval also has significant variations with similar features in previously published Delta R values for other sites in the Pacific, including southern Peru-northern Chile in southeastern (SE) Pacific, the South China Sea, Vanuatu and Papua New Guinea, with the largest magnitude of similar to 920 yr from SE Pacific. The mechanisms for these large AR variations across the Pacific during the mid-Holocene are complex processes involving (1) changes in the quantity and C-14 content of upwelled waters in tropical east Pacific (TEP) (frequency and intensity of ocean upwelling in the TEP, and contribution of Subantarctic Mode Water to the upwelled waters, which is influenced by the intensity and position of southern westerly winds), and (2) variations in ocean circulation associated with climate change (La Nina/El Nino conditions, intensity of easterly trade winds, positions of the Intertropical Convergence Zone and the South Pacific Convergence Zone), which control the spreading of the older upwelled surface waters in the TEP to the western sites. Our results imply the need for employing temporal changes in AR values, instead of constant (modern) values, for age calibration of Holocene marine samples not only for the SW Pacific sites but also for other tropical and subtropical sites in the Pacific. Crown Copyright (C) 2015 Published by Elsevier B.V. All rights reserved.