Radiocarbon evidence for mid-late Holocene changes in southwest Pacific Ocean circulation

Abstract

Variability in the southwest (SW) Pacific Ocean circulation is influenced by the changes in the South Pacific subtropical gyre and its western boundary current, the East Australian Current (EAC). The EAC plays a significant role in transporting warm, well‐ventilated, nutrient‐poor waters to more temperate higher latitudes. Recent climate changes associated with EAC intensification have led to anomalous warming in the South Tasman, with implications for marine ecosystems and environment. A clear understanding of the significance of these changes requires knowledge of past natural variability. Here we have reconstructed a 4500 year record of regional sea surface radiocarbon reservoir ages (R) and local reservoir effects (ΔR). Our results reveal the centennial‐scale variability over the last 4500 years, with R ranges as large as 390 14C yr. Older R (~410 14C yr) between 1610 to 1860 A.D. in our record, corresponding to the “Little Ice Age,” suggests a weaker influence of the EAC in the South Tasman. Between 4000 and 1900 cal years B.P., R and ΔR were significantly younger than the modern, with values of ~170 and −130 14C yr, respectively, indicating increased EAC transport of tropical waters into the South Tasman. We propose that the large R variability was influenced by strong and abrupt El Niño events which punctuated the muted El Niño–Southern Oscillation (ENSO) period in the mid‐late Holocene and enabled increased westward flow of gyre waters into the SW Pacific. The strengthening of the EAC extension appears to have been a response to the precession‐modulated ENSO‐Southern Annular Mode interactions.

New Evidence for a Mid- to Late-Holocene Change in the Marine Reservoir Effect Across the South Pacific Gyre

“…Using material from archaeological contexts comparable temporal ∆R trends have been identified in the central Pacific Gyre to those measured in coral from the southeast coast of Australia (Komugabe-Dixson et al 2016;Hua et al 2015) (Figure 2). Komugabe-Dixson et al (2016:977-78) argued that considerable ENSO variability between the present day and 1000 years ago was responsible for the increased influence of 14 C-depleted water from equatorial waters and/or from the eastern boundary of the South Pacific where high ∆R values (between 355±105 and 253±207 14 C years) were attributed to increased influence of upwelled 14 C-depleted water .…”

“…Both shell and coral proxies have identified a significant negative shift in ∆R between c. 2600 and 2250 cal BP. Komugabe-Dixson et al (2016) attributed this change to strong and abrupt El Niño events. To the far eastern boundary, a similar depression in ∆R occurred between 5180 and 1160 cal BP, which also interpreted as being caused by the increased influence of subtropical water and diminished coastal upwelling processes.…”

“…Calendar ages derived from U/Th measurements are similarly reverse-calibrated using the marine calibration curve. The new archaeological ∆R values are overlain on the black coral values from the east Australian coastline (Hua et al 2015 andKomugabe-Dixson et al 2016) and modern (pre-AD 1950) shell ∆R values from the South Pacific Gyre (Petchey et al 2008, Ulm et al 2009, both recalculated from published raw data using the online ∆R calculation tool.…”

“…More recently, information for this period of interest has become available from a sequence of paired U/Th, 14 C dates from black coral collected from the southeastern coast of Australia (Komugabe-Dixson et al 2016). Komugabe-Dixson et al (2016:976-978) noted two distinct intervals when ∆R was significantly lower; between 4000 and 3300 cal BP (∆R = -132 ± 79 14 C years) and between 2700 to 1900 cal BP (∆R = -156 ± 79 14 C years).…”

The Worldwide Marine Radiocarbon Reservoir Effect: Definitions, Mechanisms, and Prospects

“…Studies involving marine radiocarbon measurements and/or modeling have been performed in different basins, with distinct time scales and different types of samples. There have been many independent studies of oceanic paleoventilation and calculation of reservoir ages, ultimately assisting in the reconstruction of past climatic events (Andree et al, 1986;Bard, 1988;Björck et al, 2003;Bolton et al, 2016;Broecker, 1991;Broecker et al, 1988Broecker et al, , 2004Burr et al, 2015;Butzin et al, 2005;Carré et al, 2016;Cook & Keigwin, 2015;de la Fuente et al, 2015;Douarin et al, 2016;Druffel & Suess, 1983;Druffel et al, 1998;Hall et al, 2010;Hua et al, 2015;Hughen et al, 1998;Ikehara et al, 2011;Kennett et al, 1997;Kovanen & Easterbrook, 2002;Komugabe-Dixson et al, 2016;Marchitto et al, 2007;Ritz et al, 2008;Sarnthein et al, 2007;Shen et al, 2004;Stern & Lisiecki, 2013;Wündsch et al, 2016, to cite a few). Radiocarbon research in the ocean sciences community is widespread, and studies focusing on periods of extreme climate, when great variations in the MRE are likely to occur, are especially common.…”