Temporal variability in the Holocene marine radiocarbon reservoir effect for the Tropical and South Pacific

Highlights

• New ΔR for Western Pacific for ∼0–8 ka based on ¹⁴C analysis of ²³⁰Th-dated corals.

• ΔR variability of ∼200–490 yr for Western Pacific during the last ∼8 ka.

• ΔR variability of ∼300–1000 yr for coastal Eastern Pacific during the last ∼12 ka.

• ΔR variations are mainly due to large scale climate and ocean circulation changes.

• Urgent need for local/regional marine calibration curves for improved ¹⁴C dating.

Abstract

Understanding the marine radiocarbon reservoir effect (i.e., marine radiocarbon reservoir age (R) and/or correction (ΔR)) is important for the construction of robust radiocarbon chronologies for marine archives for various research areas including archaeology, palaeoecology, paleoceanography, Quaternary research and climate change studies. In this study, we examined temporal ΔR variability for the South China Sea (SCS) and the Great Barrier Reef (GBR) during the past ∼8.1 ka based on ¹⁴C analysis of ²³⁰Th-dated corals. Results show large ΔR variations of ∼410 yr and ∼490 yr for the SCS and the northern GBR for ∼5.5–8.1 ka and ∼5.5–7 ka, respectively, and a smaller ΔR variability of ∼200 yr for the SCS for ∼2–3.5 ka. Our data, together with those previously published for the Tropical and South Pacific, indicate that variability in ocean upwelling bringing old subsurface waters to the surface and/or changes in the sources (or ¹⁴C level) of the upwelled waters are responsible for seeing large ΔR variations in coastal areas along the eastern Pacific and the Tropical East Pacific (TEP) of several hundred to a thousand years mostly during the Early to Middle Holocene. ΔR variations in the central and western Pacific of several and a couple of hundred years during the Middle and Late Holocene, respectively, might be due to variability in Pacific-wide ocean circulation associated with climatic changes controlling the spread of upwelled waters from the TEP to the west. This mechanism together with local/regional effects, such as changes in ocean upwelling in the SCS resulting from East Asian monsoon variability and changes in upwelling and/or horizontal advection in the northern GBR associated with variability in the El Niño Southern Oscillation, might be responsible for the observed ΔR variations in these areas. The results of our study also indicate the need for regional marine radiocarbon calibration curves for improved radiocarbon dating of marine samples as the observed Holocene ΔR values for the Tropical and South Pacific are not fully reproduced by a recent modelling work using a 3D ocean model, which takes into account climate change effects. Ocean circulation changes were included in the model for the period of 11.5–50 cal kyr BP but possibly not considered or not well represented for the Holocene, which might explain the differences between the observed and modelled ΔR values.

Introduction

Radiocarbon (¹⁴C) chronologies constructed for marine samples are not straightforward as for terrestrial samples, which once lived in equilibrium with atmospheric ¹⁴C. Surface ocean samples (e.g., molluscs, corals, coralline algae and planktonic foraminifera) are subject to the marine radiocarbon reservoir effect (or simply the marine reservoir effect, MRE) due to the long residence time of carbon in the oceans, resulting in older ¹⁴C ages for these marine samples compared to terrestrial samples of the same age. The MRE, expressed as either marine radiocarbon reservoir age (R) or local/regional marine radiocarbon reservoir correction (ΔR), must be estimated for ¹⁴C dating and calibration of marine samples. There are several methods for reliable determination of R and ΔR values including the use of pre-bomb known-age historical marine specimens, dated corals by either band counting or U–Th, paired terrestrial/marine samples, and planktonic foraminifera in deep-sea cores. Details of these methods and their associated criteria are summarised in Table 1.

Traditionally, R and ΔR are assumed to be constant through time and their modern values are employed for age calibration of surface ocean samples (e.g., Stuiver et al., 1986; Alves et al., 2018). However, there is growing evidence that these values are not constant but vary with time. Temporal variations in these values have been documented for a number of oceans and seas (Sikes et al., 2000; Siani et al., 2001; van Beek et al., 2002; Bondevik et al., 2006; Hua et al., 2009; Skinner et al., 2015; Sarnthein et al., 2015; Lindauer et al., 2017; Toth et al., 2017). For the Pacific Ocean, large temporal R and/or ΔR variations of several hundred to almost a thousand of years were reported for a number of Holocene sites, including the Gulf of Panamá (Toth et al., 2015), southern Peru – northern Chile (Ortlieb et al., 2011; Latorre et al., 2017), central Chile (Carré et al., 2016), the South China Sea (SCS, Yu et al., 2010; Hirabayashi et al., 2019), Papua New Guinea (PNG, McGregor et al., 2008; Burr et al., 2015), Solomon Islands (Burr et al., 2015), Heron Reef in the southern Great Barrier Reef (GBR) and Moreton Bay in southeastern Queensland, Australia (Hua et al., 2015), and off the Tasman coast in southeastern Australia (Komugabe-Dixson et al., 2016).

Here we report new values for the MRE for the SCS, and the northern and southern GBR during the past ca. 8.1 cal kyr before present (BP, with 0 BP being AD 1950) derived from ¹⁴C analysis of ²³⁰Th-dated corals. Based on these new data and those previously published, we discuss temporal variations in this effect not only for our study sites but also for the wider Pacific, including the Tropical and South Pacific, and their implications for ocean circulation variability associated with climatic changes during the Holocene. We then compare these wider Pacific MRE data to the MRE evolution modelled by Butzin et al. (2017) using a 3D ocean model that takes into account effects of climate change, and discuss how our study results in improved ¹⁴C dating of Holocene marine samples for this region.